DVICO FusionHDTV5 with STB - no sound

[mudgen]

I set up my Comcast STB to output to my DVICO Fusion HDTV5 RT Gold via composite video and analog audio. I got the channel changing working, and can get video in LiveTV and recordings, but there's no sound. It's not strictly a BTV problem: I can't get sound in the DVICO software or VLC either. It appears that the DVICO should do sound straight through the PCI, but neither that nor the DVICO's analog audio line out to the mobo sound line in produce any sound. VISTA SP2, Realtek onboard sound (Dell Inspiron 530). I've connected the audio out from the STB to a pair of powered PC speakers and verified that it's pushing audio. AC3Filter is in play, but it doesn't seem to be detecting any sound signal, either.

Any suggestions? I've tried all I can think of.