Seeing as how I'm a computer science major... I'm going to try to not look stupid.
My computer has always done this thing where if I don't use it for a while it's as if my soundcard goes to sleep. My sound "chip" is a built in Ensoniq Sounblaster clone basically built into the motherboard of my Gateway. All of my power settings are turned to "always on" but I don't get why the sound would go to sleep.
The only way that I can make it come back is by playing a sound or an mp3.
Normally I wouldn't care about something this stupid, but I like to leave my Instant Messanger window open all the time and after a while, it stops making sounds when people sign on and off.