AVAudioFile format issue

[TL;DR]

Issue encountered

• AVAudioFile
  • throws when opening WAV files and MPEG-DASH files with .mp3 extension,
  • works fine with many other tested combinations of formats and extension (for example, an AIFF file with .mp3 extension is read by AVAudioFile without error).
• The Music app, AVAudioFile and ExtAudioFile all fail on the same files.
• However, previewing an audio file in Finder (select the file and hit the space bar) works regardless of the file extension.

Why do I consider this an issue?

• AVAudioFile seems to rely on extension sometimes but not always to guess the audio format of the file, which leads to unexpected errors.
• I would expect AVAudioFile to deal properly with wrong extensions for all supported audio formats.
• ⚠️ This behaviour can cause real trouble in iOS and macOS applications using audio files coming from the user, which often have unreliable extensions.

Illustration of the issue: avaudiofile-format-issue.playground

Content of the playground

In this playground project, I simply try to open various audio files for reading with AVAudioFile. File have different combinations of file formats and extensions.

• All the files have been encoded from the same WAV audio file marimba-wav.wav:
  • in AIFF using the command ffmpeg -i marimba-wav.wav marimba-aiff.aiff
  • in CAF-AAC using the command afconvert -f caff -d aac -q 127 -b 128000 marimba-wav.wav marimba-caf-aac.caf
  • in M4A using the command ffmpeg -i marimba-wav.wav marimba-m4a.m4a
  • in MP3 using the command ffmpeg -i marimba-wav.wav marimba-mp3.mp3
  • in MPEG-DASH single file using the command ffmpeg -i marimba-wav.wav -c:a libfdk_aac -f dash -single_file 1 marimba-dash.mp4
• Each file has then been duplicated and renamed with the extensions [aiff, caf, m4a, mp3, mp4, wav] and with no extension at all.

What happend when running the playground

AVAudioFile manages to open all files except

• marimba-dash.mp3: MPEG-DASH file with mp3 extension,
• marimba-wav.mp3: WAV file with mp3 extension.

Additional investigations

• The same code has been run within an iOS app, the results are the same.
• I tested a few files, it seems that when AVAudioFile manages to open the file, the audio content is properly decoded, but I did not check all the files.
• The now depreceted ExtAudioFile API behaves exactly the same as AVAudioFile.
• The Music app also fails to read the files for which AVAudiofile throws errors.
• However, preview of the audio file in Finder seems to work regardless of the extension.