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.