The README.ossctl says:
in addition to ossctl, OSS 3.99 now also automatically creates new devices
called /dev/dsp_ac3 and /dev/dsp_multich that are symlinks to the dsp device
that supports Dolby Digital AC3 and 4.1/5.1/7.1 multi-channel analog output.
But I cannot find it on my system. Can it be enabled on FreeBSD? Or is there additional documentation? I am still a bit clueless about /dev/dspx and how to work with those.