Arranged and performed in celebration of Nonesuch Records’ 50th birthday, Folk Songs pairs the trailblazing string quartet with a group of vocalists—Natalie Merchant, Sam Amidon, Olivia Charney, and Rhiannon Giddens—for a set of spare, rustic songs. The results are by turns haunting and beautiful, highlighting not only the timelessness of the music but of the stories it tells, whether it’s Merchant’s mournful “The Butcher’s Boy” or “I See the Sign,” an Amidon original that sounds 100 years old.