Leo says that it could be the sound card, and he should try to use a standalone card. If that doesn't work, then try using a USB headphone set and see if that fixes it. Otherwise it could be a driver problem. The power supply can also be the culprit, and Leo thinks that's an underestimated source of common problems in computers.
Another good idea is to have a Linux Boot Disc which would allow him to bypass the OS on his computer and boot up to another one and test to see if the hardware works. A lot of times, if the OS has issues, he'll find it right away. Leo recommends Ubuntu.