IN New York and elsewhere a “Messiah Sing-In” — a performance of Handel’s oratorio “Messiah” with the audience joining in the choruses — is a musical highlight of the Christmas season. Christians, Jews and others come together to delight in one of the consummate masterpieces of Western music. The high point, inevitably, is the “Hallelujah” chorus, all too familiar from its use in strange surroundings, from Mel Brooks’s “History of the World, Part 1,” where it signified the origins of music among cavemen, to television advertising for behemoth all-terrain vehicles. So “Messiah” lovers may be surprised to learn that the...