This one crops up frequently on the discussion groups internally at Microsoft but I'd never seen it until a couple of days ago.
On some hardware, Virtual guest machines have a very annoying habit of causing the built-in speaker on the host beeping under certain circumstances, such as on popup dialogs. This very thing happened to me this weekend as I've installed yet another virtual machine, but on a server I'd never virtualised before which runs in my loft. While this isn't annoying to me, the bigger problem is that the server is almost directly above one of my kids noses when he's trying to get to sleep. His bed is one of those raised up "bunk" type beds. Hence, his nose is some 2ft from the ceiling, and the server itself is only another 1ft up from there. Hence, me working away remotely does get very annoying for him.
The solution is simple - stopping the "beep" service and it's fully documented in the knowledgebase under KB 838671 - How to turn off the beep sound in a virtual machine under Virtual Server 2005. Essentially there are three ways, through the registry; stopping the beep service through "sc config beep"; and net stop beep.
Of course there is a fourth way which I wasn't sure why the KB didn't list - a pair of pliers or a hammer will also turn off the speaker :-)