How do I use my music library in Photos app slideshows?

Go to Photos on an iPhone, select the photos that you want to use, click share and go to slide show, tap the screen and click options, then click music and select Music library. (Note: you have to have downloaded the music or if is not downloaded but is associated with your account then you have to have good service.

Which system version do you have installed? This has changed between system versions.

And are you asking about slideshows on a Mac or a mobile device (iPhone, iPad)?

On a Mac with macOS 15.4.1 Sequoia try the following:

• Open your slideshow project.
• Click the blue note icon in the right sidebar and click "Library" and use the search field to the song you want to use.
• Click into add it to the sound track. remove the theme music that you do not want to use.

You can only use songs that are not protected by digital rights, but you can use music from Music, if you have a subscription.

But you may not be able to export the song with Music items embedded.

In some earlier versions of Photos it has not been possible at all to use your own songs you have created with garageBand or Logic Pro or saved to your music library , we have been limited to Music and the theme songs. If your Music Library is in iCloud you have to ensure that the songs are downloaded to your Mac.