I got new speakers with a built-in USB-DAC for my home computer.

Once plugged OpenBSD recognized it as an USB-audio device, so far so good. Unfortunatly I couldn’t get any sound out of it, but my computer’s sound card —which is recognised as a separate device— worked.

It turns out that by default sndiod —the system audio mixer— uses the first audio device it finds and ignores the others. To get it to use other devices you must specify them in rc.conf.local like this:

sndiod_flags="-f rsnd/0 -f rsnd/1"

I restarted sndiod like this sudo /etc/rc.d/sndiod restart, and everything now works nicely.