Saving Audio Only from Windows Movie Maker
"Is it possible to digitally record just the sound track from a movie captured in Windows Movie Maker?"
The Windows Movie Maker team anticipated this type of request and included a feature to export only the audio portion of your video as a WMA file. Instead of placing the movie clip on the Video portion of the timeline, drag it to the Audio/Music section of the timeline. When you go to publish your movie, Windows Movie Maker will ask you about audio quality settings and create a WMA, instead of allowing you to choose a video format.