Five hours before showtime, Radiohead's "Exit Music (For a Film)" blares over the empty floor of Brooklyn's Barclays Center as Marcus Mumford kicks a soccer ball with three ponytailed roadies. Then the singer jumps onstage with the rest of Mumford & Sons, experimenting with a fuzzed-out minor-key chord progression. The full touring band joins in with frantic freight-train rhythms, dissonant horns and chiming Edge-style guitar. After a few minutes, the jam fizzles, but everyone seems excited. "The world has been really supportive of our songs," Mumford says. "But there are other sounds we want to make."
Mumford & Sons are still recovering from the Grammys, where they picked up the Album of the Year trophy for 2012's Babel. "God, we're fucking knackered," says the bleary-eyed frontman, sipping coffee backstage. "When we released [2010's] Sigh No More, our manager said, 'We're probably gonna sell about 150,000 records,' because that's what Fleet Foxes sold. Since then, everything has gotten more ridiculous, fantastical and mind-fucking."
Sigh No More ended up going double-platinum – creating big expectations for the follow-up. "We were worried," says Mumford. "There was a lot in us that was like, 'That's OK, fuck it, Babel will be the disappointing second child.'" Instead, the album is another smash, with 1.92 million copies sold since September. "It's given us hope we're going to be a career band," says keyboardist Ben Lovett.
Most nights on its sold-out winter tour, the band has been using small practice rooms backstage to brainstorm ideas for the next album. "We're ready to make a departure and explore what else we can do," says Lovett. "We really want to rap," Mumford adds, laughing. 'We've just got so much to say – saying it through a melody doesn't really work for me. We've been talking to Jay-Z about it."
But first, they have to play tonight's arena gig. "These shows take a bit more out of us," says Lovett. "We have a bigger responsibility – we can't be dropping the set by 20 minutes because Marcus has tired legs." They have more dates to come this summer, including a headlining spot at Bonnaroo. "We're on the road solidly until this time next year, basically," says bassist Ted Dwane. "Then we definitely want a little break."
This story is from the March 14th, 2013 issue of Rolling Stone.