Windows Movie Maker is a great little program for what it is - and the price is right (Free). But sophisticated it is not. It offers only one Audio/Music track which usually means having to choose between having background music or a voiceover.
With a little bit of cheating however, you CAN have both and although Windows Movie Maker does not allow you to gently fade between different volume levels, this too, can be done by cheating.
So if you want to know how to use Windows Movie Maker to remove an annoying sound-track, add some nice music, a narration AND drop the background music level whilst the narrator is speaking, then this video is for you - Check it out.