Mac OS does not store theme sound sets in RAM. This sucks.
What does this mean?
It means if you're using system sounds, you can't spin down your hard drive, because every time you do anything like scrolling through the long document you're reading, it will spin up to retrieve the sound.
I suppose it might be possible to copy the sound set to a RAM disk, but I haven't tried.
I was starting to like system sounds, too.