When you use an HDMI cable, you're bypassing your Xonar Sound card. If the game you're playing does not support Dolby Digital Live, Dolby Digital, or DTS your video card is going to pass a 2.0 signal. Your receiver has a Dolby ProLogicII decoder, so you can at least set it on PLII, and get a matrix 5.1 surround.
That makes sense. I'll have to try it when I get home. Thank you!