Enable the sndiod server with:

    sysrc sndiod_enable=YES
    service sndiod start

Please note that by default sndiod is configured with a monitoring
sub-device i.e. a device through which you can record everything that
plays through sndiod:

    aucat -f snd/0.monitor -o recording.wav

Make sure you override sndiod_flags if this is not wanted.

By default the rc.d script passes '-c 0:7 -j off' to sndiod, so that
it uses all 8 virtual channels provided by OSS.  If you override
sndiod_flags consider keeping these options, so that multi-channel
audio works as you'd expect.

There is little sndio support in the FreeBSD ports tree right now.  If
your favourite port is missing support please take a look at the fork
at https://github.com/t6/freebsd-ports-sndio and
https://github.com/t6/freebsd-ports-sndio/wiki/Status which contains
patches that enable sndio support in various ports.

audio/pulseaudio-module-sndio is a PulseAudio module that allows you
to play to your sndio server.  This is useful for ports that have
PulseAudio support but no direct sndio support.