Movie Maker 2012 – audio volume drops for a second when playing the mp4 file

The movie sounds fine when I play it from movie maker, but when I save it to an mp4 file the audio seems to disappear then comes back after a second.

The audio file itself is file and it does not happen when I play the movie inside Movie Maker.
Only in the saved video file.

There is no split or transition at this point. Not sure why it happens.