I bought about one year ago my first real Apple product, mint condition PowerBook G4 15" high-res model. I like it, very much. But this bug is driving me crazy because the issue have been around since 10.5.8 came out, and what I've read from here and other forums the issue is software specific, and has something to do CoreAudio drivers. Reboot / Shutdown usually fixes it immediately but sometimes it takes couple of restarts until the sound works again. Mysterious bug this is. I shall re-install my 10.5 Leopard some day and really try to find is it some exact upgrade which messes up something with CoreAudio drivers.