Replace songs from photo album

The slideshows created from an album are essentially memory movies. To change the background music do it like it is done for a memory video:

See: Personalize your memory movies in Photos on iPhone - Apple Support

You need a Apple Music subscription to do this, if you do not like the theme songs provided:

"Change the music of a memory movie

You can change a memory movie’s music by choosing from memory soundtracks or suggested songs in Apple Music. Apple Music subscribers can also access their Apple Music library and view suggested songs based on musical preferences or the content of the memory.

1. Go to the Photos app  on your iPhone.
2. Tap Collections.
3. Swipe left under Memories to scroll through a selection of your memories, or tap Memories to view them all.
4. Tap  in the bottom-right corner of a memory to play it.
5. Tap the screen, then tap .
6. Tap , tap a song, then tap Done.
7. Apple Music subscribers can tap  to search for songs in the Apple Music library."

It is not possible to remove the background music completely. Select the least annoying song or turn down the volume.