It's the Pied Piper, Charlie Brown

It's the Pied Piper, Charlie Brown is a retelling of the story how the Pied Piper of Hamelin (portrayed by Snoopy) chased away all the mice from the town of Hamlet. The Peanuts kids substitute some characters, Snoopy being the Pied Piper. Snoopy plays concertina (squeezebox), with presumably David Benoit doing the honors. (Snoopy played accordion too a few times in the comic back in the early comics.)