To add a voiceover to your project:
Record the voiceover in a sound-editing program, such as GarageBand.
Drag the file to the audio well in the slideshow editor (to add it to a slideshow) or the Menu Info window (to add it to a menu).
If you create the voiceover in GarageBand, you can drag the file from the Audio pane in iDVD. Click the Media button, click Audio, and then click GarageBand to find the file.