The go-to track on this Christmas- and Advent-themed program is Cecilia Bartoli’s guest appearance with the venerable Sistine Chapel Choir. Wreathed in the generous acoustic, her voice makes a powerful counterpoint to the choir’s characteristically diffuse sound in Pérotin’s “Beata viscera Mariae Virginis.” The rest of the album draws on the riches of the Vatican library with its vast musical collection, focusing on polyphony from mainly the Middle Ages and the Renaissance.