Pentatonix is the hottest a capella group in American pop music. They've got the biggest selling 2014 Christmas album; and it opens with "Hark! The Herald Angels Sing". It is---and is not---your grandmother's "Hark! The Herald Angels Sing", thereby hinting at both the timelessness of Christ's birth and the ways in which each generation can (and does, and must) make the celebration come alive anew.