I've noticed that from time to time the speakers on my Macbook Pro (OS X 10.8) make a strong and harsh crackling sound instead of, or over the top of, a sound effect they are supposed to be playing. In particular, I've noticed this happens with Lotus Sametime notifications (i.e. the ping when I get a new message), but I've also noticed it happening with other applications. It only seems to happen with short sounds, not continuous ones - for example, playing videos always has perfectly smooth and error-free sound. It seems to happen most shortly after I've booted up/resumed, and goes away after a while.
What might be causing this, and how might I fix it?