How many slides do you have and how long is the audio track that you selected? iPhoto has a minimum audio length so that the slides don't go screaming past if there are too few photos for the length of audio.
Try adding a second track by creating a playlist in iTunes and selecting that playlist. Or reduce the number of slides in the slideshow.
If you've set unique timing for the various slides then you will need longer music.
To get max control of the music export the slideshow to a Quicktime movie file without music, import the file into
iMovie HD 6 and add you tracks and edit them so the music stops where you want . Then export the result to a QT file or send it to iDVD, whatever the destination is.
TIP:
For insurance against the iPhoto database corruption that many users have experienced I recommend making a backup copy of the Library6.iPhoto (iPhoto.Library for iPhoto 5 and earlier) database file and keep it current. If problems crop up where iPhoto suddenly can't see any photos or thinks there are no photos in the library, replacing the working Library6.iPhoto file with the backup will often get the library back. By keeping it current I mean backup after each import and/or any serious editing or work on books, slideshows, calendars, cards, etc. That insures that if a problem pops up and you do need to replace the database file, you'll retain all those efforts. It doesn't take long to make the backup and it's good insurance.
I've created an Automator workflow application (requires Tiger or later), iPhoto dB File Backup, that will copy the selected Library6.iPhoto file from your iPhoto Library folder to the Pictures folder, replacing any previous version of it. It's compatible with iPhoto 6 and 7 libraries and Tiger and Leopard. iPhoto does not have to be closed to run the application, just idle. You can download it at Toad's Cellar. Be sure to read the Read Me pdf file.