Every night, somewhere in the world, three or four musicians will climb on stage together. Whether the gig is at a jazz club, a bar, or a bar mitzvah, the performance never begins with a note, but with a question. The trumpet player might turn to the bassist and ask, ''Do you know Body and Soul'?'' - and from there the subtle craft of playing the jazz repertoire is tested in front of a live audience. These ordinary musicians may never have played...