Newer versions of PD have a feature called Audio Ducking, which would automatically reduce the volume of a music clip when there's activity on the narration track.
If your version doesn't have that, you'll need to use keyframes on the volume to mark the start and end of the full volume sections on the music clip. Then add two matching keyframes in between to set the lower volume level.
Here's a screenshot of the timeline and the Keyframe tool: