iMovie-audio

My iMovie projects are no longer playing back audio clips (both mp3 and voicerecordings). Any reason why this is suddenly happening? I know that my mic is working, but for some reason I can't hear anything when I go into iMovie the next day to work on my project. It's like the first half of my project works, and then the second half loses all audio, but you can still see the files are there and the audio works fine using other applications. I have 5 iMacs in my school lab and I am running into this problem on multiple machines. The only way that I can fix this problem, is to export the portion of the iMovie project that is working into a new project, and then continue editing that way. This is the only way that I can get it to work.

Please help!