Paging all would-be Herald Angels! Simon Halsey, choral director of the London Symphony Orchestra, invites you and everyone within reach of YouTube to sing along “from the comfort of your sofa.” The playlist runs from “African Noel” and “Adam Lay Abounden” to “The First Nowell” and Leroy Anderson’s jaunty “Sleigh Ride” (an instrumental). Consider forewarning the neighbors. —M.G.