5.1 Surround Sound Not Working Correctly

I just recently hooked up my Pioneer home theater system to my computer using the motherboards on board sound device via optic cables. The thing is is that all 5 speakers are playing, but it seems the rear 2 channels are playing the same as the front left and right channels. I only really notice this during a game such as TF2. Once the sound is behind my character the sound behind me plays through my center speaker. I tried right clicking the speaker in the system tray and going to configure but the configure button is gray'd out. I have the most recent drivers for the Realtek HD audio but it won't let me configure my speakers there either. Any help would be great thanks!

-Twiggy