It's not odd for audio CD's, typically they have a direct line from the drive to the sound card.. the firmware in the drive is capable of playing the CD itself.
The
cdio(1) utility in base can send this command, or extract the raw tracks which can be sent to the
audio(4) device.
Once you disable either of the sound cards, it will take precedence, this will allow mixerctl/audictl to work without manually specifying the target device.
If mplayer fails to work (..errors vs silence) then it may indeed need special handling in a driver, if it's silent it may just be muted.
In that Wiki, it seems they have a native driver that works over the ISA interface.. vs the PCI "native" interface, OpenBSD has no driver for ISA cirrus cards.