ADTS AAC file (audio/aac) from Navidrome transcoding won't play from cache — skips instantly at 0:00, no error #145

Hi! Amperfy issue BLeeEZ/amperfy#621 points here and asks for a sample file that doesn't play, so here's one.

Sample

subtle-cupcake-rhythms-navidrome-256k.aac (~4.3 MB, hosted in a gist because the API can't attach files to issues)

• Track: "subtle cupcake rhythms" by C418, from circle (2010), licensed CC BY 3.0. Transcoded from the original FLAC; no other changes.
• Produced with Navidrome's default AAC transcoding command (ffmpeg 6.1.1):

  ffmpeg -i input.flac -ss 0 -map 0:a:0 -b:a 256k -v 0 -c:a aac -f adts -
  

• Container: raw ADTS (ff f1 50 80 …), AAC-LC, 44.1 kHz, stereo, ~256 kbps
• Served by Navidrome with Content-Type: audio/aac. Amperfy stores it as .aac and passes audio/aac as the MIME type, which AudioStreaming maps to kAudioFileAAC_ADTSType.
• SHA-256: 4d2d8a2cb11a0768533332da1f609a6c11bf6c7ed4153f9391e2c9fda6473781
• ffmpeg -i … -f null - decodes it cleanly. The real length is 2:16; ADTS has no duration header, and ffprobe's bitrate-based estimate says 2:30.

What happens

1. In Amperfy, download the song with the server transcoding it to AAC.
2. Play it from the cache.
3. The track "plays" at 0:00 and immediately skips to the next one. On the server side, Amperfy reported Now Playing 5 times in 4 seconds, all at position 0.
4. Amperfy's Event Log shows no error for the failed playback.

MP3 transcodes (-f mp3) from the same server play fine. Opus is a separate issue (#137 / #139).

Environment

• Amperfy: 2.1.1 (build 22, App Store), which pins AudioStreaming 1.4.4 (Package.resolved at tag v2.1.1)
• iPhone: iPhone 16 Pro, iOS 26.5.2
• Server: Navidrome 0.61.2, ffmpeg 6.1.1

I used an AI assistant (Claude) to narrow this down and write the report. The device test and the observations above are mine.