Open the Movie Maker program. Click on "File" and "Open Project" and select the video that you want to edit. The frames of the video appear in the "Timeline" at the bottom of your screen.
Go to "File" and then "Import into Collections." Browse for the audio file that you want to add to your video. Click the file and then click the "Import" button. The music file appears in the main screen.
Select the music file, then drag-and-drop it into the part of the video you want the music to begin. Play the music file by clicking the forward-pointing arrow, which is on the left above the "Timeline." Then, when the music has finished playing, press the "Rewind" button, which is next to the "Play" button. Play the video the whole way through to make sure that the music works with the whole video.
Visit "File," then click on "Save Movie File." When prompted to save the file, choose to save it to your computer. Select "Next." Select a name and location for your video and then click "Next" again. Leave the "Movie Setting" at the "Best Quality" setting, then click "Next" to save the file. Play the video by clicking "Finish."