So another iMovie update is out iMovie 9.0.2. It fixes issues with audio playback and fixes overall stability┬áissues. So go ahead and update. Click the ´ú┐(apple) icon then click software update and update!

Post From Apple:

    This update improves overall stability and fixes an issue that could cause audio playback to be out of sync.
    This update is recommended for all users of iMovie ’11.