fix: use ADTS for AAC transcoding, temporarily exclude AAC from transcode decisions (#5167)

* fix: use ADTS format for AAC transcoding to avoid silent output on ffmpeg 8.0+

The fragmented MP4 muxer (`-f ipod -movflags frag_keyframe+empty_moov`)
produces corrupt/silent audio when ffmpeg pipes to stdout, confirmed on
ffmpeg 8.0+. The moof atom offset values are zeroed out in pipe mode,
causing AAC decoder errors. Switch to `-f adts` (raw AAC framing) which
works reliably via pipe and is widely supported by clients including
UPnP/Sonos devices.

* fix: exclude AAC from transcode decision, as it is not working for Sonos.

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/ffmpeg/ffmpeg.go

@@ -283,7 +283,7 @@ var formatCodecMap = map[string]string{
 var formatOutputMap = map[string]string{
 	"mp3":  "mp3",
 	"opus": "opus",
-	"aac":  "ipod",
+	"aac":  "adts",
 	"flac": "flac",
 }
 
@@ -339,11 +339,6 @@ func buildDynamicArgs(opts TranscodeOptions) []string {
 		args = append(args, "-f", outputFmt)
 	}
 
-	// For AAC in MP4 container, enable fragmented MP4 for pipe-safe streaming
-	if opts.Format == "aac" {
-		args = append(args, "-movflags", "frag_keyframe+empty_moov")
-	}
-
 	args = append(args, "-")
 	return args
 }

core/ffmpeg/ffmpeg_test.go

@@ -86,7 +86,7 @@ var _ = Describe("ffmpeg", func() {
 			Expect(isDefaultCommand("opus", "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a libopus -f opus -")).To(BeTrue())
 		})
 		It("returns true for known default aac command", func() {
-			Expect(isDefaultCommand("aac", "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f ipod -movflags frag_keyframe+empty_moov -")).To(BeTrue())
+			Expect(isDefaultCommand("aac", "ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -c:a aac -f adts -")).To(BeTrue())
 		})
 		It("returns true for known default flac command", func() {
 			Expect(isDefaultCommand("flac", "ffmpeg -i %s -ss %t -map 0:a:0 -v 0 -c:a flac -f flac -")).To(BeTrue())
@@ -174,7 +174,7 @@ var _ = Describe("ffmpeg", func() {
 			}))
 		})
 
-		It("builds aac args with fragmented MP4 container", func() {
+		It("builds aac args with ADTS output", func() {
 			args := buildDynamicArgs(TranscodeOptions{
 				Format:   "aac",
 				FilePath: "/music/file.flac",
@@ -186,8 +186,7 @@ var _ = Describe("ffmpeg", func() {
 				"-c:a", "aac",
 				"-b:a", "256k",
 				"-v", "0",
-				"-f", "ipod",
-				"-movflags", "frag_keyframe+empty_moov",
+				"-f", "adts",
 				"-",
 			}))
 		})

core/stream/aliases.go

@@ -80,6 +80,11 @@ func matchesCodec(codec string, codecs []string) bool {
 	return matchesWithAliases(codec, codecs, codecAliasGroups)
 }
 
+// IsAACCodec returns true if the given codec or container name resolves to AAC.
+func IsAACCodec(name string) bool {
+	return matchesCodec(name, []string{"aac"}) || matchesContainer(name, []string{"aac"})
+}
+
 func containsIgnoreCase(slice []string, s string) bool {
 	return slices.ContainsFunc(slice, func(item string) bool {
 		return strings.EqualFold(item, s)

core/stream/aliases_test.go

@@ -0,0 +1,30 @@
+package stream
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Aliases", func() {
+	Describe("IsAACCodec", func() {
+		It("returns true for AAC and its aliases", func() {
+			Expect(IsAACCodec("aac")).To(BeTrue())
+			Expect(IsAACCodec("AAC")).To(BeTrue())
+			Expect(IsAACCodec("adts")).To(BeTrue())
+			Expect(IsAACCodec("m4a")).To(BeTrue())
+			Expect(IsAACCodec("mp4")).To(BeTrue())
+			Expect(IsAACCodec("m4b")).To(BeTrue())
+		})
+
+		It("returns false for non-AAC formats", func() {
+			Expect(IsAACCodec("mp3")).To(BeFalse())
+			Expect(IsAACCodec("opus")).To(BeFalse())
+			Expect(IsAACCodec("flac")).To(BeFalse())
+			Expect(IsAACCodec("ogg")).To(BeFalse())
+		})
+
+		It("returns false for empty string", func() {
+			Expect(IsAACCodec("")).To(BeFalse())
+		})
+	})
+})