Music makes a photomontage or any video without sound a lot more interesting. With Movie Maker you can easily add songs from your personal library to any video.
In picking a song to use, consider the mood you want to set for your video, and also consider who's going to be seeing the final product. If the video is only intended for home and personal viewing, you can feel free to use any music you want.
However, if you want to share your movie publicly, or make money off of it in any way, only use music to which you own a copyright. This article will tell you more about choosing music for your movies.
To import a song into Movie Maker, choose Import audio or music from the Capture Video menu. From here, browse through your music files to find the tune you're looking for. Click Import to bring the selected song into your Movie Maker project.