Well, I spent $140 on this headset the other day, and I was really hoping it would be able to get sound from my computers line-in which I have my xbox sound going into. Im sure their's a fix out there.
You can't. The USB audio stack is independent from the rest of the system. There is no way to route sound from a mobo/soundcard to/from a USB device.
Its possible if you use programs to record the line in as it comes in, then immediatly play back to the USB device, but this induces a lot of latency in the playback.
Win8 needs a totally new audio stack to get around these types of issues...