As the lights went down and the curtain went up, the audience at TD Garden held its breath as those slow seconds passed before Mumford & Sons launched into their first song at last week’s concert. Eight yellow strobe lights in a semicircle above the band rose towards the ceiling, where twinkling lights flowed from one beam to another and pulsed along to the beat of the song. The banjo started playing, the bass drum kicked in and the band’s current #1 single, “I Will Wait,” began to play. The lights on the stage resembled a sun behind the group, and the crowd couldn’t help but sing along as the touching lyrics and upbeat melody of the song leapt from the stage. A collective “I will wait, I will wait for you” could be heard in every corner of the venue as the first of many great songs played.