Identify the audio track before separating it
A MOV file can contain video plus one or more audio tracks. There may be a commentary track, alternate language, or a track with no sound. Listen to the intended source and verify the selected track rather than assuming the first stream is the one you want.
Check the output duration against the video and look for missing sections at the beginning or end. If the clip has edits or multiple segments, compare several points across the extracted audio.