As per EmptyKube's post, here is a kernel of an idea to build on. If this starts to go somewhere, whoever wants it can write it.

Ever watch a musical and wonder what the Hell people are singing for? Ever groan when you can feel another excuse for a song coming up?

Ever wonder what non-combatants would think if they saw all these fools prancing around and singing?

So we have our POV character, who is a normal dude, but a sidekick to the Big Hero. The Big Hero breaks into song at every opportunity. Occasionally other people get sucked into the song. Once in a while the whole block is spinning and dancing and joining in with the chorus.

Meanwhile the sidekick sits on the sidelines, bored. He was at first impressed with his partner's ability to make up songs on the spot, and he was amazed at how complete strangers can join in and look like they have been practicing the song together for weeks, even though they all just met. And he has never quite figured out where all the orchestra music comes from. But most of all, he wishes it would all just stop.