So I took my PC Chips M571 motherboard out of storage to test its integrated CMI8330 once again. I encountered a minor issue with the drivers, and decided to document it here, since it might also affect the MK8330. By default, the DOS drivers mute digital CD audio input (SPDIF). There doesn't seem to be any way to change that from the graphical portion of CMMIX.COM.
However, as PARUS discovered here you can type in CMMIX /DIG:M at the command prompt to unmute this input. Once that is done, it seems to persist even after a reboot/shutdown. I'm not sure if the volume of that input can be adjusted, but simply unmuting it was enough to make digital CD audio work, at least in my case.
I've tested this with Tomb Raider and Quake, and it worked great, with both games properly playing their respective CD audio tracks. However, attempting to play a standard music CD using the supplied CDPLYR.EXE utility didn't produce any sound for some reason.