Yes, your sound card can indeed affect your fps (I assume you mean Frames Per Second, rather than First Person Shooter) - there's always been issues with soundcard drivers, for example, and if you've selected High Quality audio in the game's Settings, your rig may not be able to cope.
my issue is i play COD4 and have my frames per secondshowing in the top right corner and it runs at around 90 - 200 but when there is a lot of sound (like a airstrike or lots of people shooting in the same area) myy fps can sometimes drop to around 15 fps, so this is why i was thinking it is my sound card.
my sound at the moment is just what i bought the pc with.
Don't think it's your sound card. It could cause crashes but not alter the FPS. The framerate dropping to around 15 because there is a lot of on screen action/visuals at the times you mention which at high settings the 8600gt will struggle with.Obviously more action means more sound but that's a red herring. Lower the game settings a bit and certainly turn off AA if you have it on. I had the 8600gt and i played it at med/high with no AA and it was fairly smooth,dropping to around 25FPS if i remember right.