The card may output 5.1 but I really doubt that it would capture 5.1. I assume you are running the digtal out trough a decoder or amp? You'd have to have software in your PC to decode the 5.1 stream and save the 6 individual streams to hard disc - don't think any exists.
I'm sure there are ways of ripping 5.1 audio from a disc in the PC, but it's not something I'm into, sorry.
Capture from what source? To my knowledge, there are no legitimate 5.1 sources. You can have a 5 channel mic array, you could have 5 mics/6 mics live, and Vegas will capture all of these via sound card that has 6 discreet inputs.
No cam, no drive can output 5.1, so the only other source I can think of is taking audio from a 5.1 recording, and I doubt Sony will ever want to do this. Too many legal ramifications.