It occurred to me that diro@nixsys.bz wrote in gmane.os.netbsd.ports.i386:
> So: 
>
> cmpci0 at pci0 dev 6 function 0: C-Media Electronics CMI8738/C3DX PCI Audio 
> Device (rev. 0x10)
> cmpci0: interrupting at irq 3
> audio0 at cmpci0: full duplex, mmap, independent
> opl at cmpci0 not configured
> mpu at cmpci0 not configured 
>
> Which is able to play CDs, but not other audio. 

It should be able to play audio using the standard sound device as well
(As in: MP3s etc)

> Will commenting out the audio device entries and letting COMPAT_OSSAUDIO do 
> its stuff work? I tried setting opl0 and mpu0 manually in the kernel config 
> file and that didn't help. 

There are

opl* at cmpci? flags 1
mpu* at cmpci?

entries in the GENERIC kernel config, that should attach. COMPAT_OSSAUDIO
makes the sound device be more Linux-ish. I don't know how required it is
anymore.

> Is there any way to get the mixer on this card to work? 

The mixer is unrelated to COMPAT_OSSAUDIO, opl and mpu. What does

mixerctl -a print out?