NOTE: This is unrelated to the normal e-mail notifiaction sound in Outlook, which can be turned on or off independently.
The computer (with XP Media Center) and Microsoft Office (Student Edition) were both purchased brand new a few months ago. Installing Office was one of the first things I did, and Outlook has behaved like this from the very beginning. At first, I thought it was a default setting, and tried for several weeks to get it to stop, but couldn't find any information about this sort of thing anywhere.
Now I suspect it's somehow not Outlook, but another program that runs in the background and detects download stream. My Dad bought another Media Center computer (different vendor) and installed Office on it, and there hasn't been anything like this. If that's the case, then it probably came preinstalled on the computer.
Anybody have any ideas?