This may sound obvious, but did he check to make sure a wire didn't come loose? Also, are his speakers plugged in at the correct ports? Does this system also have a built in sound card that may be competing with his PCI sound card....Has he gone into the control panel, system, and checked for errors?
My son's computer was doing the same thing and all it was is that two of his wires that plug into the sound card were reversed.
One thing happened to mine once...after booting up, I had no sound at all! The volume control was set very high...Then I turned up the volume on my speaker...and viola! I had sound again...I don't have a clue as to why it happened, but during the boot up, the sound just didn't come with it....
Sometimes, my mp3 software (Musicmatch) desides to become the master volume controller too...but I don't really mind since I use it all the time anyway...but that can also cause a malfunction too.
