the problem ur facing migth be problem with the hardware itself.before concluding that it is h/w problem.try this following options
1.check in the bios sound options if possible try to update the bios for that system if available.
2.install win2kprofessional and download the compatible drivers from the net and update the drivers see whether the problem still persists.
3.open the system and note down the motherboard details and download the drivers for that particular motherboard from the net and check .
4.if the system is under warranty call the dealer and try to fix the problem.
thank & regards
raghu ALL THE BEST