Seven musicians entered a Chicago studio to record exactly twelve sides over the course of seven days in May 1927. Today, one of those tracks—Melancholy Blues—is currently hurtling through the cold vacuum of interstellar space aboard the Voyager spacecraft, acting as a terrestrial ambassador to whatever intelligence might find our intergalactic hi-fi. This is the strange, soaring mathematics of Louis Armstrong’s Hot Seven: a group that existed for only a week in the flesh yet managed to permanen...