You're asking about norm enforcement failure, but the story shows norms working fine: expectation communicated, pushback received, de-escalation chosen, situation resolved peacefully, albeit not to the satisfaction of the commenter. The system worked.
The fact that the teenager got defensive is indicative of his understanding of societal expectations and norms. Bending and pushing against norms is what teenagers do, and have done since the dawn of time, regardless of their cultural background and regardless of whether they're navigating the norms of a dominant monoculture or those of a multicultural society.
Your question assumes there was some breakdown that needs fixing, but the only 'problem' was mild inconvenience. When did that become evidence of cultural collapse requiring homogeneity to solve?
You still haven't explained why 'teenager won't use headphones' made you think 'multiculturalism is a problem.' For all you know, this story involved two white Christian Scots from the same cultural background. I'll gladly discuss theories about norm enforcement with you once you've explained why you deemed it necessary to inject race and culture into a story that mentioned neither.
I'm not going to entertain your 'just asking questions' routine until you do.