Honestly, guys, the 5.1 channel USB DAC is something of an albatross. The functions are all handled well, and probably better, in an AVR. Your task would be getting optical or HDMI out of your PC to your AVR. That shouldn't be all that hard to do.
I found one USB 5.1 DAC, but this is NOT a recommendation, I know nothing about it: http://www.ambery.com/usbmusocowi5.html
Notice: Dolby Digital AC3 "pass through", in other words, it doesn't decode the stream.
Here's another: http://audiotrack.net/en/usb/531
Same pass through for AC3.
So, I don't really see the point. If it's just going to take the AC3 bit stream and pass it through via optical or coax to the AVR, then what purpose does the thing serve? Just get the bit stream to the AVR directly, and let it do what it does best.
Not sure, but I think decoding the bit streams, AC3, DTS, and the high-res versions of them, probably involves licensing, which is why you won't find it on sound cards, but will in AVRs.