I am using GarageBand, when I want to make a soundtrack, that matches the slides in the slideshow.
GarageBand has a feature, where we can import a video, then replace the existing sound track with a new one. we need just to export the slideshow as a video and import it to Garageband's Movie track.
Add a movie to your project
We can trim the songs we want to use and place them exactly where we want them. Or use any of the Apple loops with cinematic sound effects, record comments, create our own music with smart instruments or software instruments.
It us fun to be able to add a dinosaur Growl or other effects, to add drum solo using a smart drummer or other smart instruments.
Details on using GarageBand are here: macOS app user guides: GarageBand for MacUser Guide - Apple Support