How to get Slideshow music from Photo to iMovie?

Create a slide show in the photos app. Add enough media to slightly exceed the duration of the song you want to use. Put your song into the slideshow. Export the slide show as a movie to your desk top. Import the movie into your iMovie project. Detach the audio from the slide show.

— Rich

There is no feature in iMovie that imports the Photos app music. When you export the slide show out of Photos the automatic music will go with it. Then, when you put the slide show into iMovie you can detach the music from the slide show and use it in your project. After detaching the music, put your cursor on the audio clip and, while pressing down, pull the audio clip straight down untill the attachment handle pulls loose. Then you can delete the video portion of the slide show.

— Rich

Great. Glad it worked for you.

— Rich