There is a good support for sndio in the FreeBSD ports tree as well. Albeit hidden behind the SNDIO build time option. It was quite the ride to get it to that point :-)
There are also now some sndio backends out there for applications not in the OpenBSD ports tree like Cava, shairport-sync, and Kodi.
I realize I'm replying to a 2 weeks old comment, so nobody will read this ever, but from your list sndio supports: streaming audio over a network, user-land mixing of audio sources, mixing of multiple audio streams at the same time,
and per application volume settings.
> Per application input/output source settings
No, but the input/output device is selectable per application via the AUDIODEVICE environment variable.
> bluetooth audio devices
OpenBSD has no bluetooth support, so no. I'm also wondering why the kernel wouldn't create audio devices from these that the userland daemon can then just transparently use? Does an audio daemon need special support for bluetooth audio devices?
There are also now some sndio backends out there for applications not in the OpenBSD ports tree like Cava, shairport-sync, and Kodi.