Mumford and Sons found surprise breakout success with their 2009 debut, 'Sigh No More,' which peaked at No. 2 on the Billboard 200 albums chart on its way to selling more than two million copies in the U.S. alone. The British folk quartet looks to match that run with its sophomore set, 'Babel' (out Sept. 24), which was recently introduced by the single 'I Will Wait.'