The voice behind “Viva La Vida,” “Speed of Sound,” et al. has a new gig. NBC‘s The Voice is tweaking its format, and Coldplay frontman Chris Martin has joined the show as key adviser, mentoring each coach’s team. The show’s new twist, called the Battles: Round Two, pits the remaining acts from each team — all previous winners and stolen artists — against one another to perform an agreed-upon song. Those episodes begin airing March 31. Martin advised the Battles: Round Two performers on vocal techniques, stage presence and tailoring songs to fit their persona and strengths.