Justin Biebers voice is improving with age, according to Island Def Jam boss Steve Bartels. The music group president said that the 17-year-olds new Christmas album Under the Mistletoe shows off how much his voice has changed over the past year. The album really shows his growth as a young man and his voice has just blossomed, Bartels told Billboard. It will pleasantly surprise many people. Its not your typical holiday album. Vocally, his balls have dropped, added Biebers manager Scooter Braun. Braun revealed that a duet with Mariah Carey on her hit 'All I Want For Christmas Is You posed a particular problem during recording, owing to her insistence that Bieber sing in her original key. EPS