[See https://github.com/deltachat/deltachat-android/issues/1531 for previous discussion.]
It would be good to be able to be more compatible by using e.g. MP3. I understand that AAC produces much smaller files for a given quality than MP3, but users should be able to trade off their data usage against usability with a wider range of clients that correspondents may use.
How about Opus? If it is supported, it is a superior format (according to its own comparison, but anyway). I think the reason for AAC is that encoding is done in the UI code which was taken from Signal and then never updated to use Opus. Opus is the only mandatory codec for WebRTC and it has been supported on Android for a while. You can already send and play Opus, the only change that should be made is to change the encoder used in the Android library call.
Opus would be great to get smaller file formats, 16kbit opus is comparable to 32kbit aac (which is comparable to 64kbit mp3).
Technically, this would require to do decoding in the core as Opus decoders are not available on all systems we want to support (iirc, missing eg. on older androids), doable, of course, but some work. This was the reason to decide for AAC.
however, Opus probably would not solve the initial issue, that some non-delta-clients cannot play an audio send by delta - i am pretty sure, that using Opus would even worsening this situation - AAC is much more common than Opus.
maybe we can add some hints to the email (that is not visible to delta-chat) as we are doing for webrtc currently.
tbh, i am really astonished that of all things Outlook resp. Windows has problems playing AAC out of the box - AAC is 23 years old and https://en.wikipedia.org/wiki/Advanced_Audio_Coding shows a really wide adaption of AAC - including Windows.
For opus encoding I proposed you a separated library time ago used by the same guy supporting ‘Call Recorder’ app for Android.
Found related feature proposal: