They do. Most headsets on the market these days support mp3/aac and the OSes support it too (can't recall if iOS supports mp3).

But does the entire chain end-to-end support sending a file unmodified? If for example there's a mixer involved so that your ringtone can interrupt your music, that music can't be piped through without decoding. Is the volume control handled in the headset or does it come earlier? That can't be applied without decoding either.

When no mixing is occurring I believe there's no decode/re-encode happening so that battery savings are realized. If there are multiple audio streams then the mixer kicks in and for that time period there would be re-encoding occurring but from an acoustic quality perspective that wouldn't matter since the multiple audio streams would already alter the perceived quality relative to the reference.

iOS supports MP3: https://developer.apple.com/library/archive/documentation/Au...