Ok, I had this problem as well and I believe that after catching the software culprit in the act (luckily is played a bit too long once) I think I have the solution!
Problem: Random sound clips and burst were playing. I would hear explosions, cars, etc... random movie ads like "the blah blah pay-per-view" etc. Just random sound in general. I also heard a "Hello" at one point.
The problem is a combination of "perfs.exe" and "indt2.sys" that are infecting your system. The thing actually playing the sound is "indt2.sys". If you open the Windows Task Manager and find it, watch and be patient. When a sound comes it will jump from like 2-3MB of Ram to over 20+MB. Additionally if you sort by memory use it you will see it is the only process that has a memory spike when one of the random sounds comes across your computer. I actually found this out by accident and after waiting a bit saw this seemed to be the problem.
Although I'm not really sure of the best sure-fire way to get rid of it, here is what I did and it hasn't shown back up in the task manager.
I used RegRun Reanimator (http://www.greatis.com/security/download.htm
) to isolate the perfs.exe file for removal. It took a few tries to get reanimator to find it, but eventually it did. I then had it remove it at the next boot. I had previously patched my registry to allow the "Take Control" option in vista. This allows you to take control of a system file and delete it. I found the registry patch here (http://www.askvg.com/add-take-owners...windows-vista/
). When my computer rebooted I navigated as fast as possible to C:\Windows\System32 and found the Indt2.sys file. I right clicked it, took control of it and deleted it. The sound seems to have stopped now.
Brief rundown of steps:
1) Make sure Vista "Take Control" is enabled by patching registry.
2) Isolate prefs.exe using Regrun reanimator.
3) Have reanimator delete prefs.exe on next boot.
4) Reboot, should get message before Vista loads saying prefs.exe was deleted.
5) Navigate to C:\Windows\System32
6) "Take Control" of Indt2.sys by right clicking and selecting "Take Control"
7) Delete Indt2.sys
8) Empty the recycle bin to flush that nasty file away.
Your annoying sounds should be gone after that. ^_^
Hope I helped! ~Matt