This is the native audio subsystem used by OpenBSD, it includes a library (libsndio) and a privsep server (sndiod). There was a massive successful undertaking to write backends for most 3rd party audio software in the ports tree, things like chromium, and it is now included upstream in SDL, OpenAL, Mozilla Firefox, mpd, etc.
I know very very little about audio frameworks, but what does sndio have over it's competition? It is clearly a simpler and likely cleaner system than PulseAudio, but what about compared to ALSA and OSS?