From e3aa8629d3ca2ee0812eef9dbfcc25aea6d759a5 Mon Sep 17 00:00:00 2001
From: Bryan Van Deusen <bryan.vandeusen@fabledsword.com>
Date: Tue, 6 Oct 2026 13:54:05 -0400
Subject: [PATCH] feat(api): deliver loudness gains to every client (M464
 #4997)

ReplayGain 2.0 values (gain to -18 LUFS, linear peak) derived from the
stored track and album loudness:

- Web: GET /api/tracks/replay-gain?ids=... (up to 200), a lookup the
  player calls for its queue, rather than a field on every TrackRef
  surface.
- Android: track_gain/track_peak and album_gain/album_peak on the sync
  views, so cached tracks level offline. Storing a measurement logs a
  track change, and an album's values moving logs an album change, both
  before the write (#2704), so caches pick the gains up.
- OpenSubsonic: replayGain on every song (album, getSong, search3,
  starred), as a JSON object and an XML element.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 internal/api/api.go                        |   3 +
 internal/api/library_sync.go               |  13 +-
 internal/api/library_sync_views.go         |  19 ++-
 internal/api/library_sync_views_test.go    |  40 +++++-
 internal/api/replay_gain.go                |  54 ++++++++
 internal/api/replay_gain_test.go           |  70 ++++++++++
 internal/db/dbq/loudness.sql.go            | 149 +++++++++++++++++++--
 internal/db/queries/loudness.sql           |  32 ++++-
 internal/library/album_loudness.go         |  58 +++++++-
 internal/library/album_loudness_test.go    |  32 ++++-
 internal/library/loudness.go               |  16 ++-
 internal/library/loudness_backfill.go      |   2 +-
 internal/library/loudness_backfill_test.go |  13 ++
 internal/library/replaygain.go             | 102 ++++++++++++++
 internal/library/replaygain_test.go        | 122 +++++++++++++++++
 internal/subsonic/browse.go                |   8 +-
 internal/subsonic/replaygain_test.go       |  58 ++++++++
 internal/subsonic/star.go                  |   3 +-
 internal/subsonic/types.go                 |  41 +++++-
 19 files changed, 797 insertions(+), 38 deletions(-)
 create mode 100644 internal/api/replay_gain.go
 create mode 100644 internal/api/replay_gain_test.go
 create mode 100644 internal/library/replaygain.go
 create mode 100644 internal/library/replaygain_test.go
 create mode 100644 internal/subsonic/replaygain_test.go

diff --git a/internal/api/api.go b/internal/api/api.go
index 65d4240b..a495c2fa 100644
--- a/internal/api/api.go
+++ b/internal/api/api.go
@@ -132,6 +132,9 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Get("/library/genres", h.handleListGenres)
 			authed.Get("/library/years", h.handleListAlbumYears)
 			authed.Get("/library/sync", h.handleLibrarySync)
+			// Before /tracks/{id} for readability; chi prefers the static
+			// segment either way.
+			authed.Get("/tracks/replay-gain", h.handleGetReplayGain)
 			authed.Get("/tracks/{id}", h.handleGetTrack)
 			// /tracks/{id}/stream is mounted above with OptionalUser so
 			// it can accept either a session or a signed token.
diff --git a/internal/api/library_sync.go b/internal/api/library_sync.go
index 96363ad1..932d7b93 100644
--- a/internal/api/library_sync.go
+++ b/internal/api/library_sync.go
@@ -10,6 +10,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
@@ -151,8 +152,12 @@ func (h *handlers) hydrateUpserts(
 		if err != nil {
 			return nil, err
 		}
+		gains, err := library.ReplayGainForAlbums(ctx, q, uuids)
+		if err != nil {
+			return nil, err
+		}
 		for _, a := range albums {
-			b, _ := json.Marshal(toAlbumSyncView(a))
+			b, _ := json.Marshal(toAlbumSyncView(a, gains[a.ID]))
 			out["album"] = append(out["album"], b)
 		}
 	}
@@ -162,8 +167,12 @@ func (h *handlers) hydrateUpserts(
 		if err != nil {
 			return nil, err
 		}
+		gains, err := library.ReplayGainForTracks(ctx, q, uuids)
+		if err != nil {
+			return nil, err
+		}
 		for _, t := range tracks {
-			b, _ := json.Marshal(toTrackSyncView(t))
+			b, _ := json.Marshal(toTrackSyncView(t, gains[t.ID]))
 			out["track"] = append(out["track"], b)
 		}
 	}
diff --git a/internal/api/library_sync_views.go b/internal/api/library_sync_views.go
index 1e8520f9..c9038c3c 100644
--- a/internal/api/library_sync_views.go
+++ b/internal/api/library_sync_views.go
@@ -18,6 +18,7 @@ package api
 
 import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
@@ -49,9 +50,14 @@ type albumSyncView struct {
 	ReleaseDate  *string `json:"release_date"`
 	CoverArtPath *string `json:"cover_art_path"`
 	Mbid         *string `json:"mbid"`
+	// AlbumGain and AlbumPeak are the album's ReplayGain 2.0 values (#4997):
+	// dB to the -18 LUFS reference, and a linear peak. Null until every track
+	// on the album is measured.
+	AlbumGain *float32 `json:"album_gain"`
+	AlbumPeak *float32 `json:"album_peak"`
 }
 
-func toAlbumSyncView(a dbq.Album) albumSyncView {
+func toAlbumSyncView(a dbq.Album, g library.ReplayGain) albumSyncView {
 	var releaseDate *string
 	if a.ReleaseDate.Valid {
 		s := a.ReleaseDate.Time.Format("2006-01-02")
@@ -65,6 +71,8 @@ func toAlbumSyncView(a dbq.Album) albumSyncView {
 		ReleaseDate:  releaseDate,
 		CoverArtPath: a.CoverArtPath,
 		Mbid:         a.Mbid,
+		AlbumGain:    g.AlbumGain,
+		AlbumPeak:    g.AlbumPeak,
 	}
 }
 
@@ -92,9 +100,14 @@ type trackSyncView struct {
 	// track is playable, which is a yes/no. The "gone since" clock is an
 	// operator concern and lives on the admin surface.
 	Missing bool `json:"missing"`
+	// TrackGain and TrackPeak are the track's ReplayGain 2.0 values (#4997).
+	// Null until the track is measured. The album's pair rides on the album
+	// view, since it changes when the album does, not when this track does.
+	TrackGain *float32 `json:"track_gain"`
+	TrackPeak *float32 `json:"track_peak"`
 }
 
-func toTrackSyncView(t dbq.Track) trackSyncView {
+func toTrackSyncView(t dbq.Track, g library.ReplayGain) trackSyncView {
 	return trackSyncView{
 		ID:          syncpkg.FormatUUID(t.ID),
 		AlbumID:     syncpkg.FormatUUID(t.AlbumID),
@@ -107,6 +120,8 @@ func toTrackSyncView(t dbq.Track) trackSyncView {
 		FileFormat:  t.FileFormat,
 		Genre:       t.Genre,
 		Missing:     t.MissingSince.Valid,
+		TrackGain:   g.TrackGain,
+		TrackPeak:   g.TrackPeak,
 	}
 }
 
diff --git a/internal/api/library_sync_views_test.go b/internal/api/library_sync_views_test.go
index bd322d23..fe627d45 100644
--- a/internal/api/library_sync_views_test.go
+++ b/internal/api/library_sync_views_test.go
@@ -8,6 +8,7 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 )
 
 // These tests pin the wire-format keys for /api/library/sync upserts.
@@ -53,13 +54,13 @@ func TestArtistSyncView_WireKeys(t *testing.T) {
 
 func TestAlbumSyncView_WireKeys(t *testing.T) {
 	a := dbq.Album{ID: validUUID, ArtistID: validUUID, Title: "Drukqs", SortTitle: "Drukqs"}
-	b, err := json.Marshal(toAlbumSyncView(a))
+	b, err := json.Marshal(toAlbumSyncView(a, library.ReplayGain{}))
 	if err != nil {
 		t.Fatal(err)
 	}
 	assertJSONKeys(t, "album", b, []string{
 		"id", "artist_id", "title", "sort_title",
-		"release_date", "cover_art_path", "mbid",
+		"release_date", "cover_art_path", "mbid", "album_gain", "album_peak",
 	})
 }
 
@@ -69,14 +70,14 @@ func TestTrackSyncView_WireKeys(t *testing.T) {
 		Title: "Avril 14th", DurationMs: 121_000,
 		FilePath: "x", FileFormat: "flac",
 	}
-	b, err := json.Marshal(toTrackSyncView(tr))
+	b, err := json.Marshal(toTrackSyncView(tr, library.ReplayGain{}))
 	if err != nil {
 		t.Fatal(err)
 	}
 	assertJSONKeys(t, "track", b, []string{
 		"id", "album_id", "artist_id", "title", "duration_ms",
 		"track_number", "disc_number", "file_path", "file_format", "genre",
-		"missing",
+		"missing", "track_gain", "track_peak",
 	})
 }
 
@@ -85,17 +86,44 @@ func TestTrackSyncView_WireKeys(t *testing.T) {
 // client's library, a missing one stays playable and fails at the speaker.
 func TestTrackSyncView_MissingReflectsTheMark(t *testing.T) {
 	present := dbq.Track{ID: validUUID, AlbumID: validUUID, ArtistID: validUUID}
-	if toTrackSyncView(present).Missing {
+	if toTrackSyncView(present, library.ReplayGain{}).Missing {
 		t.Error("a track with no missing_since must not be marked missing")
 	}
 
 	gone := present
 	gone.MissingSince = pgtype.Timestamptz{Time: time.Now(), Valid: true}
-	if !toTrackSyncView(gone).Missing {
+	if !toTrackSyncView(gone, library.ReplayGain{}).Missing {
 		t.Error("a track with missing_since must be marked missing")
 	}
 }
 
+// The gains are what Android levels playback by, offline included (#4997):
+// the track's pair rides on the track view and the album's on the album view.
+func TestSyncViews_CarryReplayGain(t *testing.T) {
+	gain, peak := float32(-3.25), float32(0.9441)
+	g := library.ReplayGain{TrackGain: &gain, TrackPeak: &peak, AlbumGain: &gain, AlbumPeak: &peak}
+	tv := toTrackSyncView(dbq.Track{ID: validUUID, AlbumID: validUUID, ArtistID: validUUID}, g)
+	if tv.TrackGain == nil || *tv.TrackGain != gain || tv.TrackPeak == nil || *tv.TrackPeak != peak {
+		t.Errorf("track view gains = %v/%v, want %v/%v", tv.TrackGain, tv.TrackPeak, gain, peak)
+	}
+	av := toAlbumSyncView(dbq.Album{ID: validUUID, ArtistID: validUUID}, g)
+	if av.AlbumGain == nil || *av.AlbumGain != gain || av.AlbumPeak == nil || *av.AlbumPeak != peak {
+		t.Errorf("album view gains = %v/%v, want %v/%v", av.AlbumGain, av.AlbumPeak, gain, peak)
+	}
+	// Unmeasured: explicit nulls, like every other optional sync field.
+	b, err := json.Marshal(toTrackSyncView(dbq.Track{ID: validUUID, AlbumID: validUUID, ArtistID: validUUID}, library.ReplayGain{}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var m map[string]any
+	if err := json.Unmarshal(b, &m); err != nil {
+		t.Fatal(err)
+	}
+	if v, ok := m["track_gain"]; !ok || v != nil {
+		t.Errorf("unmeasured track_gain = %v (present %v), want an explicit null", v, ok)
+	}
+}
+
 func TestPlaylistSyncView_WireKeys(t *testing.T) {
 	variant := "discover"
 	p := dbq.Playlist{
diff --git a/internal/api/replay_gain.go b/internal/api/replay_gain.go
new file mode 100644
index 00000000..38009db1
--- /dev/null
+++ b/internal/api/replay_gain.go
@@ -0,0 +1,54 @@
+package api
+
+import (
+	"net/http"
+	"strings"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+)
+
+// maxReplayGainIDs caps one lookup. A player asks for the tracks around the
+// one it is about to play, never a whole library.
+const maxReplayGainIDs = 200
+
+// replayGainResp is the wire shape for GET /api/tracks/replay-gain. Tracks
+// with no gain known are absent from items: no adjustment.
+type replayGainResp struct {
+	Items map[string]library.ReplayGain `json:"items"`
+}
+
+// handleGetReplayGain implements GET /api/tracks/replay-gain?ids=a,b,c (M464
+// #4997): ReplayGain 2.0 values (dB to -18 LUFS, linear peaks) for each track,
+// and for its album when the whole album is measured.
+//
+// A lookup rather than fields on TrackRef. Tracks reach the player through
+// about fifteen surfaces in two wire shapes (TrackRef and playlist entries),
+// and only the player uses the gain. One call from the player, made for the
+// tracks it is about to play, cannot be forgotten by a surface added later,
+// and costs nothing on the pages that never play anything. Android takes the
+// same values from the sync feed instead, so they are there offline.
+func (h *handlers) handleGetReplayGain(w http.ResponseWriter, r *http.Request) {
+	raw := strings.TrimSpace(r.URL.Query().Get("ids"))
+	if raw == "" {
+		writeJSON(w, http.StatusOK, replayGainResp{Items: map[string]library.ReplayGain{}})
+		return
+	}
+	parts := strings.Split(raw, ",")
+	if len(parts) > maxReplayGainIDs {
+		writeErr(w, apierror.BadRequest("too_many_ids", "at most 200 ids per request"))
+		return
+	}
+	ids := stringsToUUIDs(parts)
+	gains, err := library.ReplayGainForTracks(r.Context(), dbq.New(h.pool), ids)
+	if err != nil {
+		writeErrWithLog(w, h.logger, "replay gain: lookup", apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	items := make(map[string]library.ReplayGain, len(gains))
+	for id, g := range gains {
+		items[uuidToString(id)] = g
+	}
+	writeJSON(w, http.StatusOK, replayGainResp{Items: items})
+}
diff --git a/internal/api/replay_gain_test.go b/internal/api/replay_gain_test.go
new file mode 100644
index 00000000..150997bf
--- /dev/null
+++ b/internal/api/replay_gain_test.go
@@ -0,0 +1,70 @@
+package api
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+func TestGetReplayGain(t *testing.T) {
+	h, pool := testHandlers(t)
+	user := seedUser(t, pool, "rg1", "pw", false)
+	measured, _ := seedTrackForRemoveTest(t, h, "rg-measured", "", "")
+	unmeasured, _ := seedTrackForRemoveTest(t, h, "rg-unmeasured", "", "")
+	lufs, peak := float32(-12.5), float32(-1)
+	if err := dbq.New(pool).UpsertTrackLoudness(context.Background(), dbq.UpsertTrackLoudnessParams{
+		TrackID: measured.ID, IntegratedLufs: &lufs, TruePeakDbtp: &peak, AnalysisVersion: 1,
+	}); err != nil {
+		t.Fatalf("seed loudness: %v", err)
+	}
+	r := chi.NewRouter()
+	r.Get("/api/tracks/replay-gain", h.handleGetReplayGain)
+	get := func(query string) *httptest.ResponseRecorder {
+		req := withUser(httptest.NewRequest(http.MethodGet, "/api/tracks/replay-gain"+query, nil), user)
+		rec := httptest.NewRecorder()
+		r.ServeHTTP(rec, req)
+		return rec
+	}
+
+	rec := get("?ids=" + uuidToString(measured.ID) + "," + uuidToString(unmeasured.ID) + ",not-a-uuid")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d; body=%s", rec.Code, rec.Body.String())
+	}
+	var resp struct {
+		Items map[string]struct {
+			TrackGain *float32 `json:"track_gain"`
+			TrackPeak *float32 `json:"track_peak"`
+			AlbumGain *float32 `json:"album_gain"`
+		} `json:"items"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	g, ok := resp.Items[uuidToString(measured.ID)]
+	if !ok || g.TrackGain == nil || *g.TrackGain != -5.5 || g.TrackPeak == nil {
+		t.Errorf("measured track = %+v (present %v), want track_gain -5.5 with a peak", g, ok)
+	}
+	if g.AlbumGain != nil {
+		t.Errorf("album_gain present with no album loudness: %v", *g.AlbumGain)
+	}
+	if _, ok := resp.Items[uuidToString(unmeasured.ID)]; ok {
+		t.Errorf("unmeasured track listed; absent means no adjustment")
+	}
+	if len(resp.Items) != 1 {
+		t.Errorf("items = %v, want only the measured track", resp.Items)
+	}
+
+	if rec := get(""); rec.Code != http.StatusOK || !strings.Contains(rec.Body.String(), `"items":{}`) {
+		t.Errorf("no ids: %d %s, want 200 with empty items", rec.Code, rec.Body.String())
+	}
+	if rec := get("?ids=" + strings.Repeat("x,", maxReplayGainIDs)); rec.Code != http.StatusBadRequest {
+		t.Errorf("%d ids: status %d, want 400", maxReplayGainIDs+1, rec.Code)
+	}
+}
diff --git a/internal/db/dbq/loudness.sql.go b/internal/db/dbq/loudness.sql.go
index e4aa5f71..59d3c143 100644
--- a/internal/db/dbq/loudness.sql.go
+++ b/internal/db/dbq/loudness.sql.go
@@ -11,22 +11,13 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
-const deleteOrphanAlbumLoudness = `-- name: DeleteOrphanAlbumLoudness :execrows
-DELETE FROM album_loudness a
- WHERE NOT EXISTS (
-     SELECT 1 FROM tracks t WHERE t.album_id = a.album_id AND t.missing_since IS NULL
- )
+const deleteAlbumLoudness = `-- name: DeleteAlbumLoudness :exec
+DELETE FROM album_loudness WHERE album_id = ANY($1::uuid[])
 `
 
-// Album rows whose album has no present track left (every file missing). The
-// album row itself survives a missing file, so its loudness would otherwise
-// stay behind describing tracks that are gone; it is recomputed if they return.
-func (q *Queries) DeleteOrphanAlbumLoudness(ctx context.Context) (int64, error) {
-	result, err := q.db.Exec(ctx, deleteOrphanAlbumLoudness)
-	if err != nil {
-		return 0, err
-	}
-	return result.RowsAffected(), nil
+func (q *Queries) DeleteAlbumLoudness(ctx context.Context, albumIds []pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, deleteAlbumLoudness, albumIds)
+	return err
 }
 
 const deleteTrackLoudness = `-- name: DeleteTrackLoudness :exec
@@ -40,6 +31,56 @@ func (q *Queries) DeleteTrackLoudness(ctx context.Context, trackID pgtype.UUID)
 	return err
 }
 
+const getAlbumLoudness = `-- name: GetAlbumLoudness :one
+SELECT integrated_lufs, true_peak_dbtp FROM album_loudness WHERE album_id = $1
+`
+
+type GetAlbumLoudnessRow struct {
+	IntegratedLufs *float32
+	TruePeakDbtp   *float32
+}
+
+// The stored album values, read before a recompute so a sync change is
+// logged only when they actually move.
+func (q *Queries) GetAlbumLoudness(ctx context.Context, albumID pgtype.UUID) (GetAlbumLoudnessRow, error) {
+	row := q.db.QueryRow(ctx, getAlbumLoudness, albumID)
+	var i GetAlbumLoudnessRow
+	err := row.Scan(&i.IntegratedLufs, &i.TruePeakDbtp)
+	return i, err
+}
+
+const getAlbumLoudnessByIDs = `-- name: GetAlbumLoudnessByIDs :many
+SELECT album_id, integrated_lufs, true_peak_dbtp
+  FROM album_loudness
+ WHERE album_id = ANY($1::uuid[])
+`
+
+type GetAlbumLoudnessByIDsRow struct {
+	AlbumID        pgtype.UUID
+	IntegratedLufs *float32
+	TruePeakDbtp   *float32
+}
+
+func (q *Queries) GetAlbumLoudnessByIDs(ctx context.Context, ids []pgtype.UUID) ([]GetAlbumLoudnessByIDsRow, error) {
+	rows, err := q.db.Query(ctx, getAlbumLoudnessByIDs, ids)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []GetAlbumLoudnessByIDsRow
+	for rows.Next() {
+		var i GetAlbumLoudnessByIDsRow
+		if err := rows.Scan(&i.AlbumID, &i.IntegratedLufs, &i.TruePeakDbtp); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const getLoudnessCoverage = `-- name: GetLoudnessCoverage :one
 SELECT count(*)::bigint AS total,
        count(*) FILTER (
@@ -101,6 +142,55 @@ func (q *Queries) GetLoudnessSettings(ctx context.Context) (LoudnessSetting, err
 	return i, err
 }
 
+const getReplayGainByTrackIDs = `-- name: GetReplayGainByTrackIDs :many
+SELECT t.id,
+       tl.integrated_lufs AS track_lufs,
+       tl.true_peak_dbtp  AS track_peak_dbtp,
+       al.integrated_lufs AS album_lufs,
+       al.true_peak_dbtp  AS album_peak_dbtp
+  FROM tracks t
+  LEFT JOIN track_loudness tl ON tl.track_id = t.id
+  LEFT JOIN album_loudness al ON al.album_id = t.album_id
+ WHERE t.id = ANY($1::uuid[])
+`
+
+type GetReplayGainByTrackIDsRow struct {
+	ID            pgtype.UUID
+	TrackLufs     *float32
+	TrackPeakDbtp *float32
+	AlbumLufs     *float32
+	AlbumPeakDbtp *float32
+}
+
+// Track and album loudness for a set of tracks, for every surface that hands
+// gains to a client (#4997). A measurement from an older analysis version is
+// still delivered: it is a better gain than none until the backfill redoes it.
+func (q *Queries) GetReplayGainByTrackIDs(ctx context.Context, ids []pgtype.UUID) ([]GetReplayGainByTrackIDsRow, error) {
+	rows, err := q.db.Query(ctx, getReplayGainByTrackIDs, ids)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []GetReplayGainByTrackIDsRow
+	for rows.Next() {
+		var i GetReplayGainByTrackIDsRow
+		if err := rows.Scan(
+			&i.ID,
+			&i.TrackLufs,
+			&i.TrackPeakDbtp,
+			&i.AlbumLufs,
+			&i.AlbumPeakDbtp,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listAlbumLoudnessInputs = `-- name: ListAlbumLoudnessInputs :many
 SELECT t.id,
        (l.track_id IS NOT NULL)::boolean AS settled,
@@ -215,6 +305,37 @@ func (q *Queries) ListAlbumsNeedingLoudness(ctx context.Context, arg ListAlbumsN
 	return items, nil
 }
 
+const listOrphanAlbumLoudness = `-- name: ListOrphanAlbumLoudness :many
+SELECT a.album_id
+  FROM album_loudness a
+ WHERE NOT EXISTS (
+     SELECT 1 FROM tracks t WHERE t.album_id = a.album_id AND t.missing_since IS NULL
+ )
+`
+
+// Album rows whose album has no present track left (every file missing). The
+// album row itself survives a missing file, so its loudness would otherwise
+// stay behind describing tracks that are gone; it is recomputed if they return.
+func (q *Queries) ListOrphanAlbumLoudness(ctx context.Context) ([]pgtype.UUID, error) {
+	rows, err := q.db.Query(ctx, listOrphanAlbumLoudness)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []pgtype.UUID
+	for rows.Next() {
+		var album_id pgtype.UUID
+		if err := rows.Scan(&album_id); err != nil {
+			return nil, err
+		}
+		items = append(items, album_id)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listTracksNeedingLoudness = `-- name: ListTracksNeedingLoudness :many
 SELECT t.id, t.file_path, t.duration_ms
   FROM tracks t
diff --git a/internal/db/queries/loudness.sql b/internal/db/queries/loudness.sql
index 94aefb65..af33fb1b 100644
--- a/internal/db/queries/loudness.sql
+++ b/internal/db/queries/loudness.sql
@@ -133,11 +133,39 @@ ON CONFLICT (album_id) DO UPDATE SET
     inputs_digest   = EXCLUDED.inputs_digest,
     computed_at     = now();
 
--- name: DeleteOrphanAlbumLoudness :execrows
+-- name: ListOrphanAlbumLoudness :many
 -- Album rows whose album has no present track left (every file missing). The
 -- album row itself survives a missing file, so its loudness would otherwise
 -- stay behind describing tracks that are gone; it is recomputed if they return.
-DELETE FROM album_loudness a
+SELECT a.album_id
+  FROM album_loudness a
  WHERE NOT EXISTS (
      SELECT 1 FROM tracks t WHERE t.album_id = a.album_id AND t.missing_since IS NULL
  );
+
+-- name: DeleteAlbumLoudness :exec
+DELETE FROM album_loudness WHERE album_id = ANY(sqlc.arg(album_ids)::uuid[]);
+
+-- name: GetAlbumLoudness :one
+-- The stored album values, read before a recompute so a sync change is
+-- logged only when they actually move.
+SELECT integrated_lufs, true_peak_dbtp FROM album_loudness WHERE album_id = $1;
+
+-- name: GetReplayGainByTrackIDs :many
+-- Track and album loudness for a set of tracks, for every surface that hands
+-- gains to a client (#4997). A measurement from an older analysis version is
+-- still delivered: it is a better gain than none until the backfill redoes it.
+SELECT t.id,
+       tl.integrated_lufs AS track_lufs,
+       tl.true_peak_dbtp  AS track_peak_dbtp,
+       al.integrated_lufs AS album_lufs,
+       al.true_peak_dbtp  AS album_peak_dbtp
+  FROM tracks t
+  LEFT JOIN track_loudness tl ON tl.track_id = t.id
+  LEFT JOIN album_loudness al ON al.album_id = t.album_id
+ WHERE t.id = ANY(sqlc.arg(ids)::uuid[]);
+
+-- name: GetAlbumLoudnessByIDs :many
+SELECT album_id, integrated_lufs, true_peak_dbtp
+  FROM album_loudness
+ WHERE album_id = ANY(sqlc.arg(ids)::uuid[]);
diff --git a/internal/library/album_loudness.go b/internal/library/album_loudness.go
index 19ee5df4..7d029e0c 100644
--- a/internal/library/album_loudness.go
+++ b/internal/library/album_loudness.go
@@ -2,11 +2,14 @@ package library
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
+	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 // Album loudness (M464 #4996).
@@ -117,24 +120,46 @@ func (w *LoudnessBackfillWorker) albumPass(ctx context.Context) (AlbumLoudnessRe
 		}
 		for _, row := range rows {
 			res.Recomputed++
-			if err := storeAlbumLoudness(ctx, q, row.AlbumID, row.Digest, &res); err != nil {
+			if err := w.storeAlbumLoudness(ctx, q, row.AlbumID, row.Digest, &res); err != nil {
 				res.Failed++
 				w.logger.Warn("album loudness: recompute failed", "album_id", row.AlbumID, "err", err)
 			}
 		}
 		after = rows[len(rows)-1].AlbumID
 	}
-	n, err := q.DeleteOrphanAlbumLoudness(ctx)
+	// Listed, logged, then deleted: the same log-first order as above.
+	orphans, err := q.ListOrphanAlbumLoudness(ctx)
 	if err != nil {
+		return res, fmt.Errorf("list orphan album loudness: %w", err)
+	}
+	if len(orphans) == 0 {
+		return res, nil
+	}
+	ids := make([]string, len(orphans))
+	for i, id := range orphans {
+		ids[i] = syncpkg.FormatUUID(id)
+	}
+	if err := syncpkg.LogChanges(ctx, w.pool, syncpkg.EntityAlbum, ids, syncpkg.OpUpsert); err != nil {
+		return res, fmt.Errorf("log orphan album changes: %w", err)
+	}
+	if err := q.DeleteAlbumLoudness(ctx, orphans); err != nil {
 		return res, fmt.Errorf("drop orphan album loudness: %w", err)
 	}
-	res.Orphans = n
+	res.Orphans = int64(len(orphans))
 	return res, nil
 }
 
-func storeAlbumLoudness(
+// storeAlbumLoudness recomputes one album. The sync feed hears about it only
+// when the values clients see move: during the backfill an album's digest
+// changes with every track measured, and most of those recomputes still end
+// with no album value.
+func (w *LoudnessBackfillWorker) storeAlbumLoudness(
 	ctx context.Context, q *dbq.Queries, albumID pgtype.UUID, digest string, res *AlbumLoudnessResult,
 ) error {
+	before, err := q.GetAlbumLoudness(ctx, albumID)
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
+		return fmt.Errorf("read stored values: %w", err)
+	}
 	inputs, err := q.ListAlbumLoudnessInputs(ctx, dbq.ListAlbumLoudnessInputsParams{
 		CurrentVersion: loudnessVersion,
 		AlbumID:        albumID,
@@ -143,6 +168,14 @@ func storeAlbumLoudness(
 		return fmt.Errorf("read inputs: %w", err)
 	}
 	a := computeAlbumLoudness(inputs)
+	// Logged before the write, for the reason storeLoudness gives (#2704). A
+	// failed log stores nothing, so the digest still differs and the next
+	// pass tries again.
+	if albumVisibleChange(before, a) {
+		if err := syncpkg.LogChange(ctx, w.pool, syncpkg.EntityAlbum, syncpkg.FormatUUID(albumID), syncpkg.OpUpsert); err != nil {
+			return fmt.Errorf("log album change: %w", err)
+		}
+	}
 	if err := q.UpsertAlbumLoudness(ctx, dbq.UpsertAlbumLoudnessParams{
 		AlbumID:        albumID,
 		IntegratedLufs: a.integratedLUFS,
@@ -160,3 +193,20 @@ func storeAlbumLoudness(
 	}
 	return nil
 }
+
+// albumVisibleChange reports whether clients would see different album gains.
+// The peak is only delivered beside a loudness, so a waiting album whose peak
+// moves as tracks are measured has nothing new to tell anyone.
+func albumVisibleChange(before dbq.GetAlbumLoudnessRow, after albumLoudness) bool {
+	if !sameFloat(before.IntegratedLufs, after.integratedLUFS) {
+		return true
+	}
+	return after.integratedLUFS != nil && !sameFloat(before.TruePeakDbtp, after.truePeakDBTP)
+}
+
+func sameFloat(a, b *float32) bool {
+	if a == nil || b == nil {
+		return a == b
+	}
+	return *a == *b
+}
diff --git a/internal/library/album_loudness_test.go b/internal/library/album_loudness_test.go
index 50badf13..a943e024 100644
--- a/internal/library/album_loudness_test.go
+++ b/internal/library/album_loudness_test.go
@@ -9,6 +9,7 @@ import (
 	"testing"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 // histAt builds a histogram of n blocks all at one loudness.
@@ -162,6 +163,17 @@ func TestAlbumLoudness_Integration(t *testing.T) {
 		return res
 	}
 
+	albumChanges := func() int {
+		t.Helper()
+		var n int
+		if err := pool.QueryRow(ctx, `SELECT count(*) FROM library_changes
+		                               WHERE entity_type = 'album' AND entity_id = $1`,
+			syncpkg.FormatUUID(album.ID)).Scan(&n); err != nil {
+			t.Fatalf("count album changes: %v", err)
+		}
+		return n
+	}
+
 	second := addTrack("b", album)
 	measure(first, -10, -1)
 
@@ -173,6 +185,11 @@ func TestAlbumLoudness_Integration(t *testing.T) {
 		t.Fatalf("waiting album = lufs %v, %d/%d settled; want nil, 1/2", lufs, settled, total)
 	}
 
+	// Still no album value: clients have nothing new to read (#4997).
+	if n := albumChanges(); n != 0 {
+		t.Fatalf("a waiting album logged %d sync changes, want 0", n)
+	}
+
 	// 2. Measuring the second track changes the digest; the album is leveled.
 	measure(second, -10, -0.5)
 	if res := pass(); res.Recomputed != 1 || res.Leveled != 1 {
@@ -183,10 +200,18 @@ func TestAlbumLoudness_Integration(t *testing.T) {
 		t.Fatalf("leveled album = lufs %v peak %v, want -10 and -0.5", lufs, peak)
 	}
 
-	// 3. Nothing changed: nothing recomputed.
+	// Leveled: clients are told, once.
+	if n := albumChanges(); n != 1 {
+		t.Fatalf("leveling the album logged %d sync changes, want 1", n)
+	}
+
+	// 3. Nothing changed: nothing recomputed, nothing logged.
 	if res := pass(); res.Recomputed != 0 {
 		t.Fatalf("idle pass = %+v, want nothing recomputed", res)
 	}
+	if n := albumChanges(); n != 1 {
+		t.Fatalf("an idle pass logged album changes (now %d), want still 1", n)
+	}
 
 	// 4. A track joins (here, retagged onto this album): recomputed.
 	other, err := q.UpsertAlbum(ctx, dbq.UpsertAlbumParams{Title: "Other", SortTitle: "Other", ArtistID: artist.ID})
@@ -232,9 +257,14 @@ func TestAlbumLoudness_Integration(t *testing.T) {
 	if _, err := pool.Exec(ctx, "UPDATE tracks SET missing_since = now() WHERE id = $1", first.ID); err != nil {
 		t.Fatalf("mark missing: %v", err)
 	}
+	before := albumChanges()
 	if res := pass(); res.Orphans != 1 {
 		t.Fatalf("pass with every track missing = %+v, want 1 orphan dropped", res)
 	}
+	// Dropping the row takes the album gain away, so clients are told.
+	if n := albumChanges(); n != before+1 {
+		t.Fatalf("dropping the orphan logged %d album changes, want 1", n-before)
+	}
 	var n int
 	if err := pool.QueryRow(ctx, "SELECT count(*) FROM album_loudness WHERE album_id = $1", album.ID).Scan(&n); err != nil {
 		t.Fatalf("count: %v", err)
diff --git a/internal/library/loudness.go b/internal/library/loudness.go
index 2fd50032..78893553 100644
--- a/internal/library/loudness.go
+++ b/internal/library/loudness.go
@@ -17,6 +17,7 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 // Loudness analysis (M464 #4995).
@@ -351,10 +352,15 @@ const (
 // An inconclusive attempt leaves any existing row alone. The scan deletes a
 // row when its file changes, so a row still here describes these bytes, and a
 // value from an older method is a better gain than none until it is redone.
+//
+// A stored measurement is logged to the sync feed as a track upsert, so
+// clients that cache the library (Android) pick up the new gain (#4997). If
+// the log fails nothing is stored, and the backfill tries the track again.
 func storeLoudness(
-	ctx context.Context, q *dbq.Queries, logger *slog.Logger,
+	ctx context.Context, db dbq.DBTX, logger *slog.Logger,
 	trackID pgtype.UUID, path string, r loudnessResult,
 ) loudnessOutcome {
+	q := dbq.New(db)
 	if r.err != nil {
 		logger.Warn("loudness: analysis failed", "path", path, "err", r.err)
 		if r.inconclusive() {
@@ -388,6 +394,14 @@ func storeLoudness(
 			}
 		}
 	}
+	// Logged before the write, as reconcile does (#2704): a log that then
+	// finds the write failed costs clients one wasted re-read, but a write
+	// that lands with no log is never retried, since the track is settled, and
+	// clients would never learn its gain.
+	if err := syncpkg.LogChange(ctx, db, syncpkg.EntityTrack, syncpkg.FormatUUID(trackID), syncpkg.OpUpsert); err != nil {
+		logger.Warn("loudness: LogChange track upsert failed; not storing", "path", path, "err", err)
+		return loudnessStoreFailed
+	}
 	if err := q.UpsertTrackLoudness(ctx, params); err != nil {
 		logger.Warn("loudness: storing measurement failed", "path", path, "err", err)
 		return loudnessStoreFailed
diff --git a/internal/library/loudness_backfill.go b/internal/library/loudness_backfill.go
index 9bf05025..3379c973 100644
--- a/internal/library/loudness_backfill.go
+++ b/internal/library/loudness_backfill.go
@@ -187,7 +187,7 @@ func (w *LoudnessBackfillWorker) pass(ctx context.Context) (BackfillLoudnessResu
 						w.logger.Error("loudness backfill: track panicked", "path", row.FilePath, "panic", r)
 					}
 				}()
-				outcome := storeLoudness(ctx, q, w.logger, row.ID, row.FilePath,
+				outcome := storeLoudness(ctx, w.pool, w.logger, row.ID, row.FilePath,
 					w.analyzeFile(ctx, row.FilePath, row.DurationMs))
 				mu.Lock()
 				res.add(outcome)
diff --git a/internal/library/loudness_backfill_test.go b/internal/library/loudness_backfill_test.go
index 4a052bec..88451475 100644
--- a/internal/library/loudness_backfill_test.go
+++ b/internal/library/loudness_backfill_test.go
@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 // TestLoudnessBackfill_Integration pins which tracks a pass measures, what it
@@ -130,6 +131,18 @@ func TestLoudnessBackfill_Integration(t *testing.T) {
 			gotLUFS, gotPeak, gotHist, gotStart, gotVersion)
 	}
 
+	// Each stored measurement told the sync feed, so caching clients re-read
+	// the track and pick up its gain (#4997).
+	var logged int
+	if err := pool.QueryRow(ctx, `SELECT count(*) FROM library_changes
+	                               WHERE entity_type = 'track' AND op = 'upsert' AND entity_id = $1`,
+		syncpkg.FormatUUID(stale.ID)).Scan(&logged); err != nil {
+		t.Fatalf("count sync changes: %v", err)
+	}
+	if logged != 1 {
+		t.Errorf("re-measuring stale.mp3 logged %d track changes, want 1", logged)
+	}
+
 	// 2. A pass after a complete one is a no-op.
 	res, err = w.pass(ctx)
 	if err != nil {
diff --git a/internal/library/replaygain.go b/internal/library/replaygain.go
new file mode 100644
index 00000000..8c98ae51
--- /dev/null
+++ b/internal/library/replaygain.go
@@ -0,0 +1,102 @@
+package library
+
+import (
+	"context"
+	"math"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// ReplayGain delivery (M464 #4997).
+//
+// Clients are handed loudness in the ReplayGain 2.0 form every player already
+// understands: a gain in dB that brings the track (or album) to the RG2
+// reference of -18 LUFS, and a linear peak. A client aiming at another target
+// adds (target - ReplayGainReferenceLUFS) to the gain. The same numbers go to
+// the native API, the sync feed and OpenSubsonic's replayGain, so every client
+// levels a track identically.
+
+// ReplayGainReferenceLUFS is the RG2 reference loudness gains are relative to.
+const ReplayGainReferenceLUFS = -18.0
+
+// ReplayGain is one track's gains. Each field is nil when there is nothing to
+// say: not yet measured, silent, or (for the album pair) an album still
+// waiting on a track. AlbumPeak travels only with AlbumGain.
+type ReplayGain struct {
+	TrackGain *float32 `json:"track_gain,omitempty"`
+	TrackPeak *float32 `json:"track_peak,omitempty"`
+	AlbumGain *float32 `json:"album_gain,omitempty"`
+	AlbumPeak *float32 `json:"album_peak,omitempty"`
+}
+
+// Empty reports whether no gain is known at all.
+func (g ReplayGain) Empty() bool {
+	return g.TrackGain == nil && g.AlbumGain == nil
+}
+
+// GainDB converts an integrated loudness to its RG2 gain, to 0.01 dB.
+func GainDB(lufs *float32) *float32 {
+	if lufs == nil {
+		return nil
+	}
+	v := float32(math.Round((ReplayGainReferenceLUFS-float64(*lufs))*100) / 100)
+	return &v
+}
+
+// LinearPeak converts a true peak in dBTP to the linear amplitude ReplayGain
+// peaks are written in (1.0 is full scale), to 4 decimals.
+func LinearPeak(dbtp *float32) *float32 {
+	if dbtp == nil {
+		return nil
+	}
+	v := float32(math.Round(math.Pow(10, float64(*dbtp)/20)*10000) / 10000)
+	return &v
+}
+
+// ReplayGainForTracks returns the gains for each of ids that has any. Tracks
+// with none are absent from the map, so a lookup of a missing key yields the
+// zero ReplayGain: no adjustment.
+func ReplayGainForTracks(ctx context.Context, q *dbq.Queries, ids []pgtype.UUID) (map[pgtype.UUID]ReplayGain, error) {
+	out := make(map[pgtype.UUID]ReplayGain, len(ids))
+	if len(ids) == 0 {
+		return out, nil
+	}
+	rows, err := q.GetReplayGainByTrackIDs(ctx, ids)
+	if err != nil {
+		return nil, err
+	}
+	for _, r := range rows {
+		g := ReplayGain{TrackGain: GainDB(r.TrackLufs)}
+		if g.TrackGain != nil {
+			g.TrackPeak = LinearPeak(r.TrackPeakDbtp)
+		}
+		if g.AlbumGain = GainDB(r.AlbumLufs); g.AlbumGain != nil {
+			g.AlbumPeak = LinearPeak(r.AlbumPeakDbtp)
+		}
+		if !g.Empty() {
+			out[r.ID] = g
+		}
+	}
+	return out, nil
+}
+
+// ReplayGainForAlbums returns the album pair for each of albumIDs that has an
+// album loudness. The track fields are left nil.
+func ReplayGainForAlbums(ctx context.Context, q *dbq.Queries, albumIDs []pgtype.UUID) (map[pgtype.UUID]ReplayGain, error) {
+	out := make(map[pgtype.UUID]ReplayGain, len(albumIDs))
+	if len(albumIDs) == 0 {
+		return out, nil
+	}
+	rows, err := q.GetAlbumLoudnessByIDs(ctx, albumIDs)
+	if err != nil {
+		return nil, err
+	}
+	for _, r := range rows {
+		if g := GainDB(r.IntegratedLufs); g != nil {
+			out[r.AlbumID] = ReplayGain{AlbumGain: g, AlbumPeak: LinearPeak(r.TruePeakDbtp)}
+		}
+	}
+	return out, nil
+}
diff --git a/internal/library/replaygain_test.go b/internal/library/replaygain_test.go
new file mode 100644
index 00000000..ae18d735
--- /dev/null
+++ b/internal/library/replaygain_test.go
@@ -0,0 +1,122 @@
+package library
+
+import (
+	"context"
+	"math"
+	"path/filepath"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+func TestGainDBAndLinearPeak(t *testing.T) {
+	f := func(v float32) *float32 { return &v }
+	for _, c := range []struct {
+		lufs, want float32
+	}{
+		{-18, 0},        // at the reference: no change
+		{-9.5, -8.5},    // a loud master is turned down
+		{-23.456, 5.46}, // a quiet one up, to 0.01 dB
+	} {
+		if got := GainDB(f(c.lufs)); got == nil || *got != c.want {
+			t.Errorf("GainDB(%v) = %v, want %v", c.lufs, got, c.want)
+		}
+	}
+	if GainDB(nil) != nil {
+		t.Errorf("GainDB(nil) is not nil")
+	}
+	for _, c := range []struct {
+		dbtp, want float32
+	}{
+		{0, 1},         // full scale
+		{-6.0206, 0.5}, // half amplitude
+		{1.2, 1.1482},  // an inter-sample over, above 1.0
+	} {
+		got := LinearPeak(f(c.dbtp))
+		if got == nil || math.Abs(float64(*got-c.want)) > 0.0001 {
+			t.Errorf("LinearPeak(%v) = %v, want %v", c.dbtp, got, c.want)
+		}
+	}
+	if LinearPeak(nil) != nil {
+		t.Errorf("LinearPeak(nil) is not nil")
+	}
+}
+
+func TestReplayGainForTracks_Integration(t *testing.T) {
+	pool := newPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+	dir := t.TempDir()
+
+	measured, album, artist := seedTrack(t, pool, filepath.Join(dir, "measured.mp3"))
+	add := func(name string) dbq.Track {
+		t.Helper()
+		tr, err := q.UpsertTrack(ctx, dbq.UpsertTrackParams{
+			Title: name, AlbumID: album.ID, ArtistID: artist.ID,
+			DurationMs: 1000, FilePath: filepath.Join(dir, name+".mp3"), FileSize: 100, FileFormat: "mp3",
+		})
+		if err != nil {
+			t.Fatalf("track %s: %v", name, err)
+		}
+		return tr
+	}
+	older := add("older")
+	unmeasured := add("unmeasured")
+	f := func(v float32) *float32 { return &v }
+	for _, m := range []struct {
+		tr      dbq.Track
+		lufs    float32
+		version int16
+	}{
+		{measured, -12, loudnessVersion},
+		// A measurement by an older method is still a better gain than none.
+		{older, -20, loudnessVersion - 1},
+	} {
+		if err := q.UpsertTrackLoudness(ctx, dbq.UpsertTrackLoudnessParams{
+			TrackID: m.tr.ID, IntegratedLufs: f(m.lufs), TruePeakDbtp: f(-1), AnalysisVersion: m.version,
+		}); err != nil {
+			t.Fatalf("seed: %v", err)
+		}
+	}
+
+	gains, err := ReplayGainForTracks(ctx, q, []pgtype.UUID{measured.ID, older.ID, unmeasured.ID})
+	if err != nil {
+		t.Fatalf("lookup: %v", err)
+	}
+	if g := gains[measured.ID]; g.TrackGain == nil || *g.TrackGain != -6 || g.TrackPeak == nil {
+		t.Errorf("measured track gains = %+v, want track gain -6 with a peak", g)
+	}
+	if g := gains[older.ID]; g.TrackGain == nil || *g.TrackGain != 2 {
+		t.Errorf("older-version track gains = %+v, want track gain 2", g)
+	}
+	if _, ok := gains[unmeasured.ID]; ok {
+		t.Errorf("unmeasured track has an entry; absent means no adjustment")
+	}
+	// No album value yet (none computed): the album pair is absent.
+	if g := gains[measured.ID]; g.AlbumGain != nil || g.AlbumPeak != nil {
+		t.Errorf("album pair present before album loudness exists: %+v", g)
+	}
+
+	if err := q.UpsertAlbumLoudness(ctx, dbq.UpsertAlbumLoudnessParams{
+		AlbumID: album.ID, IntegratedLufs: f(-14), TruePeakDbtp: f(-0.5),
+		TracksTotal: 3, TracksSettled: 3, InputsDigest: "x",
+	}); err != nil {
+		t.Fatalf("seed album: %v", err)
+	}
+	gains, err = ReplayGainForTracks(ctx, q, []pgtype.UUID{unmeasured.ID})
+	if err != nil {
+		t.Fatalf("lookup: %v", err)
+	}
+	if g := gains[unmeasured.ID]; g.TrackGain != nil || g.AlbumGain == nil || *g.AlbumGain != -4 || g.AlbumPeak == nil {
+		t.Errorf("unmeasured track on a leveled album = %+v, want only the album pair (gain -4)", g)
+	}
+	byAlbum, err := ReplayGainForAlbums(ctx, q, []pgtype.UUID{album.ID})
+	if err != nil {
+		t.Fatalf("album lookup: %v", err)
+	}
+	if g := byAlbum[album.ID]; g.AlbumGain == nil || *g.AlbumGain != -4 || g.TrackGain != nil {
+		t.Errorf("album gains = %+v, want album gain -4 and no track pair", g)
+	}
+}
diff --git a/internal/subsonic/browse.go b/internal/subsonic/browse.go
index 70225084..2afb0ed8 100644
--- a/internal/subsonic/browse.go
+++ b/internal/subsonic/browse.go
@@ -185,9 +185,10 @@ func (b *browseHandlers) getAlbum(w http.ResponseWriter, r *http.Request) {
 		WriteFail(w, r, ErrGeneric, "Failed to load tracks")
 		return
 	}
+	gains := replayGains(r.Context(), q, tracks)
 	songs := make([]SongRef, 0, len(tracks))
 	for _, t := range tracks {
-		songs = append(songs, songRef(t, album.Title, artist.Name))
+		songs = append(songs, songRef(t, album.Title, artist.Name, gains[t.ID]))
 	}
 	Write(w, r, AlbumResponse{
 		Envelope: NewEnvelope("ok"),
@@ -230,7 +231,7 @@ func (b *browseHandlers) getSong(w http.ResponseWriter, r *http.Request) {
 	}
 	Write(w, r, SongResponse{
 		Envelope: NewEnvelope("ok"),
-		Song:     songRef(track, album.Title, artist.Name),
+		Song:     songRef(track, album.Title, artist.Name, replayGains(r.Context(), q, []dbq.Track{track})[track.ID]),
 	})
 }
 
@@ -386,6 +387,7 @@ func (b *browseHandlers) search3(w http.ResponseWriter, r *http.Request) {
 			WriteFail(w, r, ErrGeneric, "Song search failed")
 			return
 		}
+		gains := replayGains(r.Context(), q, tracks)
 		for _, t := range tracks {
 			album, aerr := q.GetAlbumByID(r.Context(), t.AlbumID)
 			if aerr != nil {
@@ -397,7 +399,7 @@ func (b *browseHandlers) search3(w http.ResponseWriter, r *http.Request) {
 				WriteFail(w, r, ErrGeneric, "Song search failed")
 				return
 			}
-			result.Songs = append(result.Songs, songRef(t, album.Title, artist.Name))
+			result.Songs = append(result.Songs, songRef(t, album.Title, artist.Name, gains[t.ID]))
 		}
 	}
 
diff --git a/internal/subsonic/replaygain_test.go b/internal/subsonic/replaygain_test.go
new file mode 100644
index 00000000..774093c5
--- /dev/null
+++ b/internal/subsonic/replaygain_test.go
@@ -0,0 +1,58 @@
+package subsonic
+
+import (
+	"encoding/json"
+	"encoding/xml"
+	"strings"
+	"testing"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+)
+
+// OpenSubsonic's replayGain is an object in JSON and an element with
+// attributes in XML. Clients parse both, so both are pinned (#4997).
+func TestSongRef_ReplayGainEncodesForBothFormats(t *testing.T) {
+	gain, peak := float32(-4.5), float32(0.8913)
+	tr := dbq.Track{Title: "Song", FilePath: "/m/a.flac", FileFormat: "flac"}
+	s := songRef(tr, "Album", "Artist", library.ReplayGain{TrackGain: &gain, TrackPeak: &peak})
+
+	b, err := json.Marshal(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var m map[string]any
+	if err := json.Unmarshal(b, &m); err != nil {
+		t.Fatal(err)
+	}
+	rg, ok := m["replayGain"].(map[string]any)
+	if !ok {
+		t.Fatalf("JSON has no replayGain object: %s", b)
+	}
+	if rg["trackGain"] != -4.5 || rg["trackPeak"] == nil {
+		t.Errorf("JSON replayGain = %v, want trackGain -4.5 and a trackPeak", rg)
+	}
+	if _, ok := rg["albumGain"]; ok {
+		t.Errorf("JSON replayGain carries albumGain with no album value: %v", rg)
+	}
+
+	x, err := xml.Marshal(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(x), `<replayGain trackGain="-4.5" trackPeak="0.8913"></replayGain>`) {
+		t.Errorf("XML lacks the replayGain element: %s", x)
+	}
+}
+
+func TestSongRef_NoReplayGainWhenUnmeasured(t *testing.T) {
+	s := songRef(dbq.Track{Title: "Song", FilePath: "/m/a.mp3"}, "Album", "Artist", library.ReplayGain{})
+	if s.ReplayGain != nil {
+		t.Fatalf("unmeasured song carries replayGain %+v", s.ReplayGain)
+	}
+	b, _ := json.Marshal(s)
+	x, _ := xml.Marshal(s)
+	if strings.Contains(string(b), "replayGain") || strings.Contains(string(x), "replayGain") {
+		t.Errorf("unmeasured song encodes replayGain: %s / %s", b, x)
+	}
+}
diff --git a/internal/subsonic/star.go b/internal/subsonic/star.go
index b2b333f5..7d717a3a 100644
--- a/internal/subsonic/star.go
+++ b/internal/subsonic/star.go
@@ -191,6 +191,7 @@ func loadStarred(ctx context.Context, q *dbq.Queries, userID pgtype.UUID) ([]Son
 	if err != nil {
 		return nil, nil, nil, err
 	}
+	gains := replayGains(ctx, q, trackRows)
 	for _, t := range trackRows {
 		album, err := q.GetAlbumByID(ctx, t.AlbumID)
 		if err != nil {
@@ -200,7 +201,7 @@ func loadStarred(ctx context.Context, q *dbq.Queries, userID pgtype.UUID) ([]Son
 		if err != nil {
 			return nil, nil, nil, err
 		}
-		songs = append(songs, songRef(t, album.Title, artist.Name))
+		songs = append(songs, songRef(t, album.Title, artist.Name, gains[t.ID]))
 	}
 
 	albumRows, err := q.ListLikedAlbumRows(ctx, dbq.ListLikedAlbumRowsParams{
diff --git a/internal/subsonic/types.go b/internal/subsonic/types.go
index 19ffb63d..2414fe60 100644
--- a/internal/subsonic/types.go
+++ b/internal/subsonic/types.go
@@ -9,6 +9,7 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 )
 
 // IDs on the wire are the bare UUID string. Subsonic endpoints know from
@@ -180,6 +181,20 @@ type SongRef struct {
 	BitRate     int      `json:"bitRate,omitempty" xml:"bitRate,attr,omitempty"`
 	IsDir       bool     `json:"isDir" xml:"isDir,attr"`
 	Type        string   `json:"type" xml:"type,attr"`
+	// ReplayGain is OpenSubsonic's replayGain (M464 #4997): Minstrel's own
+	// measurements, so third-party clients that level by ReplayGain do it
+	// from the same numbers the Minstrel apps use. Omitted until measured.
+	ReplayGain *ReplayGain `json:"replayGain,omitempty" xml:"replayGain,omitempty"`
+}
+
+// ReplayGain is the OpenSubsonic replayGain object: gains in dB to the
+// -18 LUFS ReplayGain 2.0 reference, peaks as linear amplitude. A field with
+// no value is omitted.
+type ReplayGain struct {
+	TrackGain *float32 `json:"trackGain,omitempty" xml:"trackGain,attr,omitempty"`
+	AlbumGain *float32 `json:"albumGain,omitempty" xml:"albumGain,attr,omitempty"`
+	TrackPeak *float32 `json:"trackPeak,omitempty" xml:"trackPeak,attr,omitempty"`
+	AlbumPeak *float32 `json:"albumPeak,omitempty" xml:"albumPeak,attr,omitempty"`
 }
 
 type SongResponse struct {
@@ -284,7 +299,10 @@ func albumDetail(a dbq.Album, artistName string, songs []SongRef) AlbumDetail {
 	}
 }
 
-func songRef(t dbq.Track, albumTitle, artistName string) SongRef {
+// songRef takes the track's gains as an argument rather than looking them up,
+// so no response that lists songs can leave them out by forgetting a step:
+// each caller fetches them once for its whole list (replayGains).
+func songRef(t dbq.Track, albumTitle, artistName string, g library.ReplayGain) SongRef {
 	s := SongRef{
 		ID:          uuidToID(t.ID),
 		Parent:      uuidToID(t.AlbumID),
@@ -313,9 +331,30 @@ func songRef(t dbq.Track, albumTitle, artistName string) SongRef {
 	if t.Genre != nil {
 		s.Genre = *t.Genre
 	}
+	if !g.Empty() {
+		s.ReplayGain = &ReplayGain{
+			TrackGain: g.TrackGain, AlbumGain: g.AlbumGain,
+			TrackPeak: g.TrackPeak, AlbumPeak: g.AlbumPeak,
+		}
+	}
 	return s
 }
 
+// replayGains fetches the gains for a list of tracks in one query. A failed
+// lookup yields no gains rather than failing the response: the songs still
+// play, just unleveled, which is how they played before gains existed.
+func replayGains(ctx context.Context, q *dbq.Queries, tracks []dbq.Track) map[pgtype.UUID]library.ReplayGain {
+	ids := make([]pgtype.UUID, len(tracks))
+	for i, t := range tracks {
+		ids[i] = t.ID
+	}
+	gains, err := library.ReplayGainForTracks(ctx, q, ids)
+	if err != nil {
+		return map[pgtype.UUID]library.ReplayGain{}
+	}
+	return gains
+}
+
 // coverArtID returns the album UUID as the cover-art key. getCoverArt uses
 // the album row to find art either in cover_art_path (when the scanner sets
 // it) or via sidecar lookup in the album directory, so emitting the id