To use only the audio in a QuickTime movie as a soundtrack:
Make sure the menu to which you want to add the audio is showing in the iDVD window.
With your pointer over the menu (and with no button or text object selected), press Command-I to open the Menu Info window.
The Menu Info window has controls for modifying the menu.
Locate the QuickTime movie you want to use.
The movie can be in the Movies section of the Media pane or stored elsewhere on your computer or a connected server.
Drag the movie to the audio well (shown below) in the Menu Info window.
The icon in the audio well changes to the QuickTime icon.
Drag the Loop Duration slider to adjust how long you want the audio file to play.
The duration you choose applies to both the audio file and any movie you’ve added to the menu as background. The maximum duration shown on the slider is the length (up to 15 minutes) of the longest media (movie, audio, or slideshow) you have added to the menu background. If you have added just the audio, the slider indicates how long the audio clip is.
Adjust the volume of the sound by dragging the Volume slider to the right or left.