Here’s an album to transport you to a medieval Advent service. It comes complete with Gregorian chant, antiphons, and readings from the Bible—sung unaccompanied in Latin, or, as in “Vox de celo,” accompanied on a psaltery with a dramatic declamation over the top. Exquisite instruments such as bells, drums, and medieval harps add to the magic of what must have been a powerful occasion in the religious calendar. La Reverdie and I Cantori Gregoriani sing and play with complete conviction and an impressive sense of drama.