Logitech Z640 - Center Speaker is not responding

[addiCtjOkaH]

Hey, I just received my Logitech Z640 5.1 Speakers but my center speaker doesn't play. It works when it is in matrix mode but that is only to play 5.1 when the sound card is only a 4.1. I have a Hercules Game Theatre XP and on my Game Port/Drivers, it shows support up to 7.1 but my center speaker isn't playing. I have the Game port on the 5.1 Speaker setup but still nothing.

Any suggestions?
Thanks