Sync issues are pesky. Possibly something in the rendering process caused it. Nothing to do but try a few things.
As a workaround, you can do a Modify/Detach Audio and that will separate out the embedded audio into an independent clip. Then you can slide the audio clip slightly to sync it with the video. Start by moving it to the right slightly, and then hold down the Option key and tap the < key to nudge it to the left in one-frame increments. This procedure does not work for nudging to
the right.
Another thing you can do after detaching the audio, is to Control-click on the audio clip and select Show Speed Editor from the drop down menu. That will reveal a speed adjustment slider handle in the upper right hand corner of the audio clip. Sliding to the left speeds up the clip; sliding to the right slows it down. Try making a slight adjustment to the speed of the audio clip by sliding the handle. You can do this same procedure with the video clip as well, to slightly speed it up or slow it down to match the audio.
You can also try importing your original clip into QuickTimePlayer and see if it plays in sync there. If so, export it out from QT and see if it plays in sync.
Another thing you can try, and this may be the best way, is to convert your clips to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to beMp4/AAC. Re-rendering often cures audio issues. Then import the converted clips into imovie.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich
