If you are using Hardware Encoding, try switching it off.
Member @jonray had this tip for another member with a similar problem to yours:
This has happened to me before. I found a workaround was to advance the audio track forwards by 7 frames (I presume your audio is on a separate track). It seems out of synch in the preview but synched in the export.
You can test just a portion of your video by using the “timeline to source” feature.
See also this thread: