In the novel, Crispin learns how to play a music-pipe or recorder from Bear. Since the story is set in the 14th Century, this would be a Renaissance or early Renaissance recorder. A recorder is a woodwind instrument in the flute family. Before the 18th century, recorders were often referred to as the common flute.
When Crispin first tries to play on the recorder, his efforts prove a dismal failure. It is Bear who teaches the young boy how to maneuver the instrument.
He began by instructing me about the pipe's holes- the stops, he called them- and the way to shape my mouth around the blowing end, how to shift my fingers, how to make different sounds.
With Crispin on the recorder while Bear sings, the pair are able to make their way through their perilous journey without attracting undue suspicion about their motives or mission.