Greetings marilynroth,
Thank you for using Apple Support Communities, it's great to have you here.
We want to help you get Photos working as expected, but we're not sure exactly what you are experiencing. If you'd like to reply back to this message, let us know what you mean when you say you're only getting the "canned version" of songs. In the meantime, we think the information in this resource will be helpful: Create slideshows in Photos on Mac - Apple Support
Specifically, we'd want to note this section:
1. To select music for the slideshow, click the Music button , click the down arrow next to Music Library, then select a song or songs.
2. Choose Theme Songs from the pop-up menu to see music included with Photos, or choose Music to see music from your Music library. Drag the songs you selected to change the order they play in. To delete a song, select it and press Delete.
3. 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.
We hope this information is helpful!
Have a great day.