When editing a video, Movie Maker lets you choose between Storyboard view, and Timeline view. With Storyboard view, you see just a still frame of each photo or video clip. Timeline view separates the clips into three tracks, one for video, one for audio, and one for titles.
When adding music or other audio to your video, switch from Storyboard view to Timeline view by clicking on the Show Timeline icon above the edited movie. This changes the editing setup, so that you can add an audio track to your video.
Drag the song icon to the audio track and drop it where you want it to begin playing. After a song is in the timeline it's easy to move around and change the starting point.

