Or not. I've most certainly known people who would say something without having any gun in a situation like that. They exist, believe me. I'm not one of them btw. I would have endured it for a couple minutes and left. If I had a neighbor who blasted loud ass music constantly though, I might eventually go over and say something to them (or maybe I'd call cops, not sure) - and I wouldn't be armed when I did it. In a functioning society that's supposed to be possible to do.
But the whole point was that he needed a gun to make it a level playing field. Who would enter a situation that needs a gun to be level if the alternative is enduring listening to loud music you don't like for a few minutes?
Or maybe he didn't need a gun to make it a level playing field after all.
I don't think there's anything strange about someone asking another person to turn their music down without being armed. But I have a feeling this particular person wouldn't have done it without his gun. I do believe he shot because he legitimately felt threatened, but I don't believe it was because he had a shotgun pointed at him. I think he had preconceived notions about the people he was confronting that put him in a heightened state of fear and impulse reaction. You yourself said that it's natural to have these kinds of prejudices against certain groups. He said they were listening to thug music, which means he probably viewed them as thugs. His girlfriend knew him to routinely complain about this music but never actually confront anyone over it, meaning that in this case he was pushed over his normal threshold and probably not very calm.
I find his narrative, that these people responded to "could you please turn your music down" with "I'm going to kill you" as very bizarre. I could see them saying he'd better leave or else they'd do something to him, to get him to leave them alone. But telling him they'll going to kill him outright for politely asking to turn the music down strikes me as just comically implausible. Maybe they really were this ridiculously hostile and stupid, it just doesn't seem like a likely story. I could believe it more if the situation escalated where Munn didn't back off when he should have, and/or if he said something incredibly offensive. But that'd put a lot more on him. And it'd be much more of a process he could have left and not a sudden danger that he had no choice but to react with than by shooting at them.