Goron, could indeed be a sound card, if this is integrated into your motherboard, it could be costly as you would have to replace the board and likely have to replace the processor and memory unless its relativly new, ( the curse of moores law), id look at your sound settings by clicking the little speaker icon near your clock if they are all around the same level its unilikely that this is your problem, next check would be that you speaker sockets are all connected properly (im sure your done this), and finaly test the speakers, use some headphones in your speaker socket, if you are getting the same result then try unplugging your soundcard (if its not integrated) and pushing it back in (obviously after pooptung down the PC and earthing your self to avoid static damage) cards can occasionaly come unseated especially if not screwed down. If you do need to replace the card then its not to bad news.
as you can pick them up for about £15.
Hope this helps
Dene