I can only make a guess here. I would assume that you have onboard sound, and that other than the sound dying, the computer is behaving normally. I assume also that when you reboot you don't actually switch off the speaker power. This would lead me to think that there is a hardware problem in your onboard sound circuit. If I'm right, then I think your best bet would be to disable the onboard sound and fit a pci sound card. You can get them for less than £10, although for quality sound you would need to spend about twice that amount.