Thanks for the suggestion. The nice command wouldn't accept a plus after the number 15. So instead of entering:
nice -n +15 mozilla-firefox
as you suggested. I had to enter:
nice -n 15 mozilla-firefox
Unfortunately, this did not help.
I've since come to realize that this problem only occurs when I use compiz-fusion. Also, it seems to occur no matter what media player I am using.
If you have any other ideas, I'd be glad to hear them.