Audio on imovie not working?!?! HELP!?

okay so the audio on my imovie 08 isn't working. I've used it before and it worked fine but when i tried to use it today and record a vid (using the built in camera on my imac) it recorded the vid fine but the audio wasn't there. The volume on my computer is fine and the audio on vids that i've recorded previously work. It's just this vid that isn't working.I also recorded 3 testers after that vid to make sure the audio wasn't working. it . Help me please!!