The problem is caused by the audio codec isse. The Reason why there is no sound or missing audio in MP4 imported in iMovie, iMovie for iOS iPhone/iPad and the Fix Though, MP4 is a supported video format for iMovie, many users find that MP4 video has no sound or audio after imported in iMovie (iMovie 10, iMovie 10.1.9, iMovie for iOS iPhone/iPad included).
In the early post, TechiSky introduces the Best Video Format for iMovie, which is MP4, MOV, MPEG-2, AVCHD, DV, HDV, MPEG-4, H.264.
