Great question — answering as the UphillTech team:
You have a dependency situation where snd_sb8 is either not installed or the installed version is incompatible with snd_driver.ko . You can see this here: KLD snd_driver.ko: depends on snd_sb8 - not available or version mismatch So, this is probably why /dev/mixer is missing. Could you find out what audio chip you have, what does lspci report ? You need to fix the sound driver, I guess you probably compiled something from mismatched sources.
Full answer by thecarpy: https://unix.stackexchange.com/a/791352 (CC BY-SA 4.0).
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