Windows Movie Maker/Adding audio
Click on the TASKS tab below HELP. Under CAPTURE VIDEO click on IMPORT AUDIO OR MUSIC.
Browse for your music or audio, and click IMPORT.
Adding it to the video
Simply drag and drop the thumbnail representing the audio onto the timeline. To move it around, drag it left and right along the timeline while the cursor is in the shape of a hand. You can split and set the volume for individual audio tracks, so if you want to have different parts of the audio louder or quieter you can split the audio into chunks and individually set the volume for each. You can shorten audio by grabbing the end of the audio on the timeline and dragging it when the cursor changes to two red arrows. You can make audio fade in or out, which is useful when it the audio is longer than your video otherwise the music wouldn't end gracefully.
You can also set the volume for video clips' audio. Right click on the bars in the AUDIO row (the added audio/music's row is titled AUDIO/MUSIC).