what happened to my slideshow music?
After I updated the OS the I photo slideshow music changed
MacBook Air (13-inch Late 2010), iOS 9.3.1
Which version of iPhoto are you using? Or are you now using the new Photos for Mac application?
Photos for Mac uses different Theme Music for the slideshows from the iPhoto Theme Music.
See: How Photos handles content and metadata from iPhoto and Aperture - Apple Support
Photos preserves slideshows. If a slideshow theme isn't available in Photos, it uses the default Photos slideshow theme.