OS X :: Internal Speakers On IMac - Jarringly Loud Noises?

A year ago next month I bought the best iMac on the market at the time, as souped up as it could possibly be. I have not had any problems with it until now. Last night, my computer began suffering an incredibly strange problem with the internal speakers (I don't own any external ones). It all began when I opened GarageBand: noise was suddenly 10x louder, I tried to turn the volume down and the clicks that usually sound off when I work the volume were not only louder, but different clicks. The only way I could stop the jarringly loud noises was to mute the computer entirely. When I exited out of GarageBand, everything returned to normal.

I thought that was the end of the problem until today. I was on Youtube watching a music video, and in the midst of changing my status on Yahoo messenger, I heard another extremely loud click noises. These clicks don't sound like they're coming from some kind of malfunction in the computer. They sound like the computer actually means to make this sound, like it's stored in the computer's library of sounds. That went away, but the volume was again messed up. It was way too loud and in between turning it down, the volume suddenly disappeared altogether! Without being muted, my computer was not making any noise, although the music video should have still been playing. Only time seemed to solve the problem, as the sound later returned (I exited messenger and re-opened it as I had exited GarageBand--but I wasn't sure if this is what caused the sound to return).