In Windows 7 Ultimate SP1 x64, a few days ago I noticed that the startup sound did not play. I then realized that none of the system sounds are playing (btw, all non-system sounds were playing, so I don't think it is a driver issue). However, after randomly pressing on a program which prompted UAC, I realized that it played and other system sounds played. Not long after, they would not play when they were supposed to.

Why is this happening?