Pentatonix became massive by adding their signature a cappella touch to songs from all over the pop spectrum—and their similar approach to Christmas music has made the quintet synonymous with the holiday season. They put modern spins on classics like “Sleigh Ride,” which gets a beat-boxing-assisted makeover, and “Little Drummer Boy,” which has its brightness turned up; they bring unexpected selections like Leonard Cohen’s “Hallelujah” and Fleet Foxes’ “White Winter Hymnal” into the seasonal canon; and they spread the joy around by collaborating with big-voiced superstars like Jennifer Hudson and Kelly Clarkson.