Half of my voice overs just stopped making sound

In iMovie, I need a project for class, and my voice-overs for some of my clips just stopped working. I can still see them in the project viewing screen, but when I try to play them, they don't make any sound. At the same time, some of the VO clips made sound in the correct locations. When I try to re-record the VO's, they show up on the audio level viewer, but still make no sounds. Does any body know how to fix this problem?