QuickTime Player is an audio player, why wouldn't it be? However, the underlying complaint is that Music doesn't claim to be capable of handling mp4. Which is

QuickTime Player is an audio player, why wouldn't it be? However, the underlying complaint is that Music doesn't claim to be capable of handling mp4. Which is basically true; the "m4a" and "m4v" that Music does claim to support are restricted subsets of mp4. Should Chrome handle files with a .heif extension if they contain AV1? That's pretty much the same question.