What happened when they were in the boat?why did mole cry out? Chapter 7

At one point, Mole notices Rat leaning out the edge of the boat, transfixed with something Mole cannot see. Rat then begins to cry, saying that he heard a beautiful melody that mad him extremely happy. A moment later, Mole hears the song, a tune coming from pan pipes. Entranced, they follow the music to a small island in a narrow stream.