"The Sing-Off" is a four-night event premiering Monday, December 14 (8-10 p.m. ET) that features a one-of-a-kind battle of voices that continues on December 15 and 16, culminating with a live finale on December 21 (8-10 p.m. ET each night). The show will feature the country's top eight a cappella groups performing popular songs in a way that viewers never heard them before. There's no lip-synching, back-up bands or safety net. They'll be singing for America's vote with the winner walking away with the ultimate prize -- an Epic Records/Sony Music recording contract. (Source: NBC)