To add a soundtrack to a slideshow:
Double-click the button for your slideshow on the DVD menu to open the slideshow editor.
Click the Media button to open the Media pane, and then click Audio.
Click GarageBand to find songs in GarageBand, or iTunes to find music in your iTunes library.
Drag a playlist or a song from the Audio pane to the audio well (shown below) in the slideshow editor.
You can also drag audio files from another folder on your hard disk or from a connected server to the audio well. The audio well icon changes from a speaker to one that corresponds with the type of file you added.
The slide duration is automatically set to “Fit to Audio” to ensure that the slideshow and soundtrack finish at the same time. If you want to manually change how long each slide stays onscreen, choose the number of seconds from the Slide Duration pop-up menu.
When you do this, the slideshow and music may become out of sync, so it’s important to preview the slideshow to make sure you like it. Click the Preview button at the bottom of the iDVD window, which brings up the iDVD remote control. Click Exit in the remote control to return to the slideshow editor.
Click the Return button to exit the slideshow editor.