You know after 7 years of this audio syncing, or missing, nonsense in iMovie I am starting to feel like a ^(&*!@# broken record about this problem. Sad thing is, the solution is the freaking same as it was back in 2003! Now that iMovie 11 is here we STILL have the audio not syncing to the video? Come on Apple how is it that Final Cut NEVER... EVER has this issue yet it continues on in iMovie and comes back like a bad penny?!?!?. Rant over.

Extract, or detach, the audio and make sure the audio is in sync.

Extract-Audioweb