I have a Sony 5.1 system hooked up to my computer by optical cable to my P5Q-E, but I'm not getting any sound from the center channel. Soundmax doesn't give me an option to raise the volume on any speakers except the two front ones, and there is no way to set Vista to a 5.1 setup that I am seeing. I know I am missing something kind of obvious, I just can't figure out what it is 