How do I change the soundtrack for a slideshow in Photos for Mac?

You can have your own music.

Create a slideshow project

1. In the Photos app on your Mac, select the photos you want in your slideshow.
2. You can add or remove photos later (see “Add or remove photos and text in a slideshow,” below).
3. Choose File > Create > Slideshow > Photos.
4. Click the Slideshow pop-up menu and choose New Slideshow.
5. Type a slideshow name in the Slideshow Name field, then click OK.
6. To reorder photos, drag photos into the order you want them in the bottom of the window.
7. To select a theme, click the Themes button , then click a theme.
8. You can change the theme at any time. The theme you choose determines text position, fonts, and the transitions that appear between slides. The pan and zoom effect, also known as the Ken Burns effect, is available only for the Ken Burns theme.
9. To select music for the slideshow, click the Music button , click the down arrow next to Music Library, then select a song or songs.
10. 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.
11. 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.
12. To change how long each slide appears, click the Duration button , then do any of the following (not all options are available for all themes):

• • Set the slideshow to match the length of the selected music: Select Fit to Music.
  • Set a specific display time for the slideshow:Select Custom, then drag the slider to specify how long the slideshow should play.
  • Set a transition for the entire slideshow: Select the Transition checkbox, then choose a transition type from the pop-up menu.
  • Set slides to fit the screen: Select the “Scale photos to fit screen” checkbox.

1. To see a preview of the slideshow, click the Preview button.

Very much agree with previous posting.

That is why I have used Keynote or iMovie to make personalized Slideshows.

Notation: Keynote will only allow using Non DRM music wherein iMovie will allow using Digital Rights Managed music

EDITED

When you pick a song for your slide show be sure the default music is deleted. I just made a slide show and also found it played the default music but when I checked the panel I saw that the default was on the list at the top. Once deleted it played the music I chose.

"This cannot be right, can it?"

From my limited experience; I think that's right.