In Photos 3 on High Sierra you may want to try this method:
How to turn an Album into a Slideshow and preserve the Sorting, 2019 Version
As to the music: Because we can save a slideshow project as a video, which would remove all digital right from the audio files in the sound track, we cannot use audio files for the sound track, that have digital rights attached. Usually you can use songs purchased from the iTunes Store, or songs you have ripped from an (unprotected) CD, songs you have recorded yourself, for example by creating a song arrangement in GarageBand.
- But you cannot use the songs you saved from Apple Music
- Or very old iTunes purchases, that are still protected (file kind: protected -aac- audio file)
- or some songs from a CD, for example older Sony CDs.