Hi TruthGoodnessBeauty,
Thanks for using Apple Support Communities. We understand you have some questions about playing music with slideshows. We're happy to help. To help isolate this issue, try adding a different song to the slideshow using these steps:
" 1. Click the Music button
2. Click the down arrow next to Music Library,
3. Select a song or songs.
4. Choose Theme Songs from the pop-up menu to see music included with Photos, or choose Music to see music from your Music library.
5. Drag the songs you selected to change the order they play in. To delete a song, select it and press Delete. "
See more about those steps here: Create slideshows in Photos on Mac - Apple Support
Let us know if this happens with all songs, or only some. We'll go from there.
Thanks.