What program are you guys running your sound through? You DO realize that stuff like winamp, basic media sounds, and the like aren't 5.1 sources, and thus the center channel won't play? Also, most dvd player software that comes free with drives won't allow 5.1 playback.
I'd be willing to bet money that your soundcard is fine, and your problem is that nothing you're using is 5.1 compatable for output.