Stock front speakers don't always work. Whats the problem/how do I fix it?

I own a 97 Toyota Camry xle and my front speakers work on occassion but when I hit a speed bump or the song I'm listening to has too much bass the front speakers stop working. The left doesn't work more than the right. the front right speaker has a 50% chance of working when I drive while the front left has a 10% chance. what could be the problem? the speakers work but is it a fuse? or something I can fix? I just took an auto course so I know basic automotive terminology and principles but any help would be appreciated.