the strangest sound error you've ever heard of. PLEASE HELP!

a few months ago i noticed while i was sitting in class that every time my powerbook generates any sort of alert it automatically changes my volume to a certain level (the specific level changes every time i restart my computer). this gets really annoying when i need my volume to be at a certain level and whenever there is any sort of alert i have to change the volume back to the level that i had it before the alert sounded. any help would be greatly appreciated. apple couldn't figure out anything about the problem and its not just isolated to my own account. thanks a lot!