Repasting this from where i originally posted it. chosing these options fixed my sync problems in final cut express. should work for imovie though too.
Both worked for me and ended up with in sync audio. but from what i have read the 2nd way listed works more reliably.
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From that article i was able to put together the following. These are two export settings that appear to work. Good to know after you Video has been rendered after upload to youtube, it takes another 1-2 hours before the option will show up to watch the HD version
Apply the Deinterlace filter mentioned above to your time line then export as follows.
Export HD for Youtube as mpeg 4
File -->Export --> Quick Time Convertion
Set the Format to be mpeg-4
Click the options button
Set the file format to be mp4
On the video tab set as follows
Video format = H.264
Data Rate = 2500
image size = 1280x720 HD
On the Audio Tab
Date Rate = 192 KBPS
Ok setting and save the file, Ready for upload to You tube. My test worked fine but i have read that people have had audio sync problems with this. if you have that problem use the 2nd way listed next.
Export HD for Youtube as Quicktime
File -->Export --> Quick Time Convertion
Set the format
Format = Quicktime Movie
Click Options
Click Settings
Set the following options leave everything else as is
Compression type = Mpeg-4 Video
Frame Rate = Set this to whatever your original frame rate was.
Quality = Best
Click ok
Click Size
Set to HD 1280x720 16:9
Click ok
Click sound
Format = AAC
rate = 44,100
Thats it. This one worked for me but took longer to output from FCE.