there's a few synthesis libs they could be using: max, ChucK, SuperCollider, Impromptu, CSound

there've been some relevant threads e.g. https://news.ycombinator.com/item?id=7571082

https://news.ycombinator.com/item?id=8344827

(i've never tried any, still geting to grips with engines like DSI mopho, korg ms2k, blofeld, minibrute, bass station etc

That may be unlikely, considering a few of the things you mentioned are proprietary or not available for OpenBSD. What is more likely is that Alexandre created something new, which makes sense considering he also created the sndio API discussed in the article.

yes, that's not unlikely, and very interesting if its the case.

the openbsd audio stuff is very interesting. and sort of odd for that platform to have.

pureData would be my first guess.