Working with the Video Soundtrack and Other Audio Tracks
GarageBand imports the audio track of the movie as a single track and adds it to a podcast using the name Video Sound. You can edit this track in all the same ways that you can edit any other audio track. You can add any number of effects to various parts of the track to enhance the sound; you can control the volume levels and mix with other tracks.
You can also record additional tracks directly using GarageBand or add other audio files or projects. By default, a podcast episode in GarageBand includes Male Voice and Female Voice tracks (both with radio effects added to them) as well as a Jingles track and a Radio Sounds track. With the exception of the Radio Sounds track, which is created as a Software Instrument track, all are Real Instrument tracks that you can record into or use for importing existing audio files from your iTunes library or other GarageBand projects using the iLife Media Browser.
You can use or not use any of these tracks. If you want to use only the audio that was recorded along with the video, you can simply not record or work with any of the tracks other than the Video Sound track. You can also record narration to go along with the video or add music to your podcast (or both).