Megadeth‘s Dave Mustaine celebrated his 51st birthday on Sept. 13 by playing a concert to a packed house in Buenos Aires, Argentina. After playing their last song of the night, Mustaine was surprised with a birthday cake onstage by two lovely ladies, while the crowd sang ‘Happy Birthday’ to the legendary frontman in both English and Spanish. Megadeth founding member and bassist David Ellefson led the crowd in singing ‘Happy