The Music section
This section of the Album displays the music files in a folder on your hard drive. To use a file drag it onto the Music track or another audio track on the Movie Window Timeline.
The Music folder: The wav, mp3 and other audio files come from the folder named at the top left of the section. Other music files can be added to the section by storing them in this folder. You can also select a different folder to be the source of the section.
Click here for information on using background music in your movie.