Youve got 5.1 or 7.1? If yes, then i find the problem strange since i have 5.1 myself.
If you got just stereo or 2.1 then more sound will have to come through less speakers, so that might be the case.
I got a creative soundblaster xfi titanium fatal1ty gamer championship soundcard with an optic fibre cable mounted to my phillips 5.1 set, and found no problem whatsoever.