I'm sure that there might be some software DSP on your computer that can do something to translate those two channels into the six on the sound card...

I was able to do exactly this with the integrated sound on my motherboard, with a certain version of the drivers that didn't work correctly. I had audio from Winamp playing in Dolby Digital through my new receiver, and it sounded great, until it started clicking out. I guess the Dolby signal wasn't 100% compatible. And with the new drivers, it's still not, but it no longer forces the stereo source to become six channels, either.

But it was just an experiment. I wasn't really looking to have all sounds come from all speakers at all times.