Audio Prob in my system...How to rectify tat?

My system has got sub woofer but sound s not audible.Woofer works well in another sys.i thought prob s in sound file and so i installed XP again but still it continues.If sound card s damaged thn there ll be no sound but there s some sound.It also affects my Headphone,its too not working....Plz tell me where the problem is?