This vocal quartet planned to call it quits with their second album of music devoted primarily to the medieval chants of 12th-century nun Hildegard von Bingen. Eventually, the group carried on—and even found a place for itself in the realm of contemporary music, by recording Pulitzer Prize winner David Lang’s Love Fail. But this set of beguiling music has remained a tentpole of Anonymous 4’s discography, thanks to the beautiful singing on highlights like “O ignis spiritus paracliti” and “O felix anima.”