I found this on the help menu by searching add music.
Burn the music you want to a CD.
Play a CD during a presentation
Insert the CD into the CD-ROM drive.
On the Slides tab in normal view, click the slide you want the music to start playing on.
On the Insert menu, point to Movies and Sounds, and then click Play CD Audio Track.
If you want the music to repeat, select the Loop until stopped check box.
Under Play CD audio track, in the Track boxes, set the starting and ending track number. (To play only one track or part of a track, put the same number in both boxes.)
In the At boxes, set the starting time for the starting track and ending time for the ending track. By default, the start time is zero and the end time is the total number of minutes for the ending track.
Click OK, and when prompted, do one of the following:
To play the music automatically when you go to the slide, click Yes.
To play the music when you click the CD icon , click No.
To adjust settings for when to stop the music: on the slide, click the CD icon , right-click, and on the shortcut menu, click Custom Animation.
In the Custom Animation task pane, click the arrow on the selected item in the Custom Animation list, and then click Effect Options.
On the Effect tab, under Stop playing, do one of the following:
To stop the music on mouse-click of the slide, select On click (the default).
To stop the music after this slide, click After current slide.
To keep the music playing for several slides, click After, and then set the total number of slides the music should play on.
Notes
You need to insert the CD into the CD-ROM drive to play it during the presentation.
To change the settings for track number and start and end time, right-click the CD icon , and on the shortcut menu, click Edit Sound Object to display the Sound Options dialog box.