A fresh arrangement can transform a well-worn tune. That’s clear when you listen to the bold, zippy versions of “White Christmas,” “Amazing Grace,” and “Nutcracker” (a cheeky twist on Tchaikovsky) performed by this men’s a cappella crew. The crooners alternate between singing out and supporting some A-list guests, providing a rich tapestry of beats, hums, and chords to underpin star turns by Paul McCartney and Colbie Caillat. Through it all, their harmonies are nothing less than heavenly.