How do I change slideshow music

Yes, I am using Sonoma. But I had to search for songs that were not greyed to. I can use the songs I created myself in GarageBand by singing along to my guitar, or songs I recorded and saved, but not Apple Music.

it has been changing frequently between system versions. It is only since macOS Ventura that I can use again the songs I recorded and arranged myself in GarageBand. And I used to be able to use the songs ripped from CD or purchased, as long as they had been downloaded to the Mac and have not been removed by the "optimize Storage" feature.

I forgot to mention - we have to be signed in to our media account in the Music.app to be able to use the purchased music. Have you checked, if you are still signed into your iTunes account? I could load more of my songs in a Photos slideshow after signing again into the account (from the Account menu in Music). To find songs I used a smart playlist with the rule combination showing all purchased songs on this Mac. I forced a download for the songs that I wanted to use.

Create slideshows in Photos on Mac – Apple Support (UK)

"Note: If you don’t see music from your Music library listed, close Photos, then open Music and sign in if you haven’t already. With Music open, open Photos again, then select your slideshow. Click the Music button  and choose Music from the pop-up menu to see your music list appear."

When you create a slideshow project with "File > Create > Slideshow Apple" look at the controls at the right border of the new slideshow.

Click the button with the blue note to access the songs from your Music Library or use the theme music.

The field "Selected Music" will show initially some Theme Music; after you add a song of your own, you can erase the theme music from this field.

Some songs may be greyed out, when you want to select a song. The songs need to be stored locally (downloaded from iCloud", and the songs must not be protected by digital rights.