When you make a preset, you can choose what sound loop or song you want to start when it activates. Just go to the preset editor, and tap on the Intro settings button, which looks sort of like an arrow leading into the assignment circles:
You'll see a row labelled "Choose Song..." if you want to access your music library. Be aware that you'll need to grant Lightbow access to your music library, so be prepared for an alert that looks like this:
After you click OK, you'll see a standard Apple Music picker where you can use the search field or navigate through your library to find the perfect song.