It's the Pied Piper, Charlie Brown(2000)
It's the Pied Piper, Charlie Brown is a retelling to Sally by Charlie Brown of the story how the Pied Piper of Hamelin (portrayed by Snoopy) chased away all the mice (changed by Charlie Brown from rats because Sally is terrified of rats) from the town of Hamlet. The Peanuts kids substitute some characters, Snoopy being the Pied Piper Beagle with his contract being for a year's supply of dog food. Snoopy plays a concertina (squeezebox), with presumably David Benoit doing the honors. (Snoopy played accordion too a few times in the comic back in the early comics.) The special breaks Peanuts tradition in that it shows adults, who speak audible language (unlike most other Peanuts videos in which adults are only heard off stage speaking in "wah wah" sound of a muted trombone); although this is not the first special to do this.