fade out music at the end of a photo memory on MacBook pro

This cannot be done directly in Photos.

You have to edit the finished video in a different app. Two of the apps that are coming for free on a Mac can do it. iMovie or GarageBand.

• If you want to use iMovie, see: Fade audio in iMovie on Mac – Apple Support (UK)
• Or, in GarageBand, you add the video to a Movie Track, then add a fade-out to the Master Track volume automation and write the changed audio back to the original movie: Add a movie to your project in GarageBand on MacOfficial Apple Support › guide › garageband › mac