To bring the house down means to perform so successfully as to cause the audience to erupt in applause or cheers for a long time, which is what the crowd would do if the boys went back out.
Consider the statement "I thought the crowd might applaud uproariously for an extended period of time if the boys didn't go back out there.". Based on this "I thought the crowd might bring the house down if the boys didn't go back out there." a true statement? Yes, no, or I don't know?