I lived in a 2010/2011 build flat for a while that had really good sound insulation... was while I was a student and the other flat on the same floor had loud parties every other night. The parties could be heard in the hallway by the elevator... but as soon as you went inside the flat and closed the door - it was inaudible. Also good for me in the other direction, as I like having a decent surround sound system and my subwoofer wasn't subtle... no complaints. Later lived in another flat with similarly good noise insulation. Then went to Switzerland and the neighbours in both flats could hear even normal volume conversations and even called to police on us more than once for volume levels even I wouldn't complain about and I'm fussy... both were new builds. All depends on the place... just have to investigate thoroughly and test as much as you can. For what they were... they were good... but now I really struggle to live in anything other than a detached house... I'm making even more noise and we have cats.