If you’ve been having trouble getting your iPhoto slideshows to play music from iTunes, then follow the directions below and your problems will be solved!
This solution uses the iTunes feature “reorganize files in the folder ‘iTunes Media'” to correct compatibility issues between iPhoto and iTunes. No data will be lost in the process.
More info can be found on this Apple forum if you’re interested
Open iTunes and navigate up to the main menu bar
Click File > Library > Organize Library
Click the check box next to “Reorganize files in the folder ‘iTunes Media'”
Tip: After reorganizing your iTunes Library, be sure to relaunch iPhoto if it has remained open during this process.
You’re done! Go back to your iPhoto slideshows, set the music from your iTunes library that you want to play, and click start!