I learned far more than I needed to about soundproofing when we were having noisy neighbour issues (we moved in the end) and the tests for modern developments are an absolute joke.
I think music is, possibly, a more pervasive sound than background traffic. That's the kind of thing you slowly tune out of as being repetitive and there all the time. With music it's not constant, and so you get used to periods without it, so when it is loud it's more immediately noticeable. YMMV of course!
But I'm certainly in the "There's been a venue there for ages, surely you knew that" camp, and people should be even more aware that if they're buying modern flats near venues then it's going to be an issue.
Wasn't there someone who bought a flat in Ramsay Gardens then complained about the Tattoo? There was definitely someone a few years back bought a flat next door to the Sheep Heid and complained and they had to change the beer garden opening times, as well as finish the pub quiz earlier.