I believe it's a requirement for properties converted into flats to carpet the upstairs flat, as a sound-proofing aid, though I'm not sure how legally enforceable this is. Or even if it ever happens. Or if it even makes a difference.
In our old property I'd frequently pop round to ask the teenage son and his mates to kindly stop MCing through microphones at full blast along to Dizzee Rascal, and this worked for all of the first five minutes it took them to decide I was an irrelevance and start again. I was very pleased one night to see him taken out of the house in handcuffs for some non-related incident, but unfortunately he returned the next day.
So yes, back to the point, try talking first, as that link suggested, but if I'd had my chance again, I probably would have got the council involved.