Re: syncing up music with video on a mac?
All I use is iMovie as well. And I basically load-in the audio file, and then hand-sync it to the video. Never had any problems.
I will warn you that uploading to YouTube after doing your project in iMovie requires different settings that YouTube has spelled out in a FAQ. Definitely look that up because once you set this up right, every video you upload will be nice and clear.