How can I embed synced lyrics into a MP3 or AAC file which also shows as synced Lyrics in Apple Music.app ?

[porg]

You possibly have the knowhow, hence I dare to ask here

Concrete Question

• What exact metadata format must synced lyrics comply to in MP3 and AAC (M4A) files so that Apple Music on macOS shows them indeed as synced lyrics?
  • ID3 in MP3?
  • XMP in M4A container with AAC audio codec?
  • What text character encoding? (I'd prefer UTF-8)
  • What timecode: Milliseconds or MPEG frames?

My failed experiments

With Kid3 I added this metadata to the ID3 metadata of a WAV file

• Lyrics Nr.1, Language: "ger" (German), Title "Viennese German", as audible in the song, UTF8
• Lyrics Nr.2, Language: "ger" (German), Title "High German" (the translation for those not familiar with the Viennese dialect), UTF8
• Synchronized Lyrics, Language: "ger" (German), Title "Viennese German", milliseconds as time units, UTF8
  • During playback in Kid3 the editor/preview windows jumps to the corresponding timestamped lines, so the timing is correct.
  • I also managed to export those synced lyrics as a .LYR file, should that be needed.

Convert WAV to MP3 and AAC and preserve all metadata

• The basic ffmpeg -i input.wav output.mp3 || output.m4a carries over most metadata (Artist, Title, Album) but not all:
  • .mp3 (MP3 file with metadata in ID3 which ideally is preserved from the WAV file as-is)
    • Lyrics "Viennese German" and "High German" are copied over, but the synced lyrics not.
  • .m4a (MP4 container with AAC audio + metadata as XMP)
    • No Lyrics copied at all.

Also manual postprocessing attempts with Kid3 failed

• Copy tag from WAV and paste to MP3 or M4A file
• Inspecting this mediainfo frontend "InvisorLite" on macOS shows me this:
  • M4A
    • Lyrics: Shows the Viennese German version, the High German version is dropped.
      • Those M4A/XMP allow only one lyric record possibly?
      • In the source I had two lyric tags, both using language code "ger", only differing in the titles
      • So overall: In ID3 this seems to work, but in M4A/XMP only one seems to "make it"
    • SYLT - Synchronized lyric/text: Viennese German, whether only the title makes it, or also the content, I tried to determine by also inspecting with a Hex editor
      • Certain unique text phrases which are identical in German and Viennese German, occured only once. So this is a strong proof that this file has no 2 static lyrics + 1 synced lyrics., but rather only 1 static lyric. Unless ofc the "atomization" of the lyrics makes even ASCII text hard to find.
  • MP3
    • Contains a Field Lyrics Which shows: "High German: all High German lines // Viennese German: all Viennese German lines".
      • Seems like a complete representation
    • Shows no Synced Lyrics at all

Thanks for any directions!

[porg]

Preliminary but seemingly quite final research answer

• No, Apple Music on macOS as of 2024-01 does not support synchronized lyrics for local audio files from ripped CDs or self-produced files.
• This feature is only available for songs from the Apple Music subscription service.
• → More details