From Apple TIL file # 22180: "In an MPEG movie, the audio and video are usually multiplexed into a single track. As of May 1998, QuickTime 3 can extract the video track but not the audio track of an MPEG movie. What this means is that when you open an MPEG movie in MoviePlayer 3 and then Export the movie to the QuickTime Movie format, the resulting movie will not have an audio track. Of course, you can still open and play MPEG movies with audio and video in MoviePlayer as long as you have the QuickTime MPEG extension installed. Apple hopes to add this functionality to QuickTime in the future."