Sound problem in Tiger

Hello. I am having a problem with the sound from my iMac G5. Basically the speakers work and if I play a tune from iTunes its fine. If I try the volume control it works as well (makes the blop sound). I noticed the problem when I was playing Ghost Recon. Main music is ok but the sound effects (such as people talking and shooting are not present). The problem manifests itself as well in quicktime. There is no sound whatsoever in quicktime but VLC works ok... What can I do? Can someone please help? Do I need to give you more information???