My neighbour has a machine running XP that no longer makes Windows 'event' sounds, such as the Startup/Shutdown sounds. I've checked all the obvious things. If I go in to edit the sound scheme, I can see the sounds are assigned to the event, I can click the little 'play' button and it plays the right sound. It just won't make any noise on its own.
What really got me stumped was that if I click the volume icon in the system notification area and then start changing the volume, rather than a ding sound at various volumes from the speakers, all I get is a beep from the internal speaker.
It's an HP machine of some sort, with Realtek audio.