Sound out of sync after iMovie '11 upgrade

I have a project nearly an hour long made in iMovie '09. I just upgraded to iMovie '11 and now the sound is out of sync with the picture. It gets progressively worse the longer into the movie you go. I discovered this when I went to do more work on the project. The sound and picture were perfectly in sync in iMovie '09. I don't understand how Apple can release an upgrade with no warning that it will destroy all your previous work. What happens now?