Sync problems can occur when the video is recorded on a different device than the audio. This has nothing to do with your export settings.
One thing you can try is to manually sync the audio. Import your previously exported movie into a new iMovie project. Do a Modify/Detach Audio to place the audio into the time line as a clip separate from the video. Then do a Modify/Split Clip of the audio clip at the point where it starts to go out of sync. You can then slide the audio clip slightly to bring it into sync. Or, you can control click on the audio clip and select Show Speed Editor from the pop up menu. That will reveal a speed control slider handle in the upper right hand corner of the clip. Slide it slightly to the right to slow it down and to the left to speed it up. You can do the same thing with the video clip to try to get the two in sync.
Sometimes re-encoding can cure sync problems. Try re-encoding your exported video with the free download, Handbrake.
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 see if the video and audio are synced.
-- Rich
