Gary Rosenzweig looks at two ways to convert video files to mp3 audio files so you can listen to them on audio-only devices. The first method uses QuickTime Pro and iTunes. The second method uses FFMpegX.
Update: MacMost viewer Nick pointed out that you can also use GarageBand 3 to do this. Just drag and drop the video into GarageBand, delete the video track, and export as a .mp3 file.