Q: what happened to my slideshow music?

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.