Add background music to your movie using the Add music command on the Home tab. Note that you can add music or add music from the current point. The Music Tools tab will allow you to set end points, fade in and fade out, and set music volume. Music will play along with the audio track on video clips, so you may want to control volume in these circumstances. From the video tools tab it is also possible to control the volume of the audio on your video clips. At some points you might want to emphasize background music, at others lower background music and increase video soundtrack.
Note that Movie Maker will trim music to fit the length of the video. It will not leave music playing when the video has ended. Above all, it is recommended that you assemble your project in its entirety, including title and credit clips, before adding music. Adjusting music to fit additionally added assets is very difficult.
Video Clip Audio Volume
Select the video clip you wish to control the audio volume on by clicking it in the story board window. Select Movie Tools. On the far left of the Movie Tools tab find the volume slider, and next to it fade-in and fade-out tools. Make adjustments as you wish. Remember that the volume slider affects the entire clip, not just a portion of it.
Here is a screen video that demonstrates the various techniques appropriate to using background music and controlling soundtrack audio in Windows Live Movie Maker.