Three folk acts descended upon the Belly Up Tavern on Sunday night, drawing a sold-out crowd that greeted them as if they were leather-clad rock stars. The enthusiastic, unlikely reception served as proof that you never can predict what kind of music people will fall in love with.

The first band, San Diego’s The Silent Comedy, worked up the crowd with their reliably visceral performance. Properly gob-smacked by the high-energy set, the crowd teemed with excitement, and looked to the second band — Australia’s The Middle East — with heightened optimism.

Unfortunately, the band failed to meet those expectations. The Middle East’s set started out strongly enough, with their finger-picked minor chords breaking out into some well-executed, slow-motion freakouts. Still high off The Silent Comedy’s set, the crowd went nuts for the first few songs — you would have thought they’d just heard “Pour Some Sugar On Me,” not some non-descript, slack-paced Americana.

Before long, the band’s bleary material proved far too mellow for its own good, and the crowd’s attention began to wander. By the set’s midway point, the sound of the audience talking was as loud as The Middle East’s dull, meandering compositions. The performance verged on flatlining — even some of the band’s members appeared to have fallen asleep standing up — but the group pulled it together for their last two songs, thanks in no small part to being joined onstage by Mumford & Sons for their finale.

Some time later, Mumford & Sons returned for their headlining set. Opening with “Sigh No More,” off their breakout album of the same name, the London band recaptured the good will that The Middle East had squandered. The four-piece kept their elegant harmonies front and center, backing them with stand-up bass, furiously picked banjo and the pounding kick drum at lead singer/guitarist Marcus Mumford’s feet.

One song after another, the band gave the audience what they craved: Celtic-tinged fighting/drinking songs that crescendoed into rousing shout-alongs. The crowd went suitably nuts for the moving “I Gave You All,” the newly penned “Nothing is Written,” and the radio hit “Little Lion Man.” In between songs, the amiable band offset their serious material by trading quips and entertaining the crowd with humorous anecdotes.

Despite the lackluster Middle band, Mumford & Sons’ live precision and good-natured showmanship got the evening back on track. It was a little strange to see the huge Solana Beach crowd gush over some scrappy folk acts, but Mumford & Sons and The Silent Comedy deserved every bit of the praise.