Every material our there has pores in it, and thus contains air. With some materials out there, like lead, these pores are so incredibly small, they are hardly noticeable under a microscope. But they are there.
Right, I hate the way that metal things aren't airtight. Oh wait... they are.
You are right of course. This does not apply to metals. Sorry about that.
It'll be all the steelwork, not the concrete, that's conducting all the sound-waves through Doug's apartment block.
Correct here also. Most concrete is reinforced concrete, with metal nets in it. I don't agree though that it's the only reason, because you'll see the same effect with masoned walls without any metal in them. But I would agree that, for concrete walls that do contain metal it'll be a lot worse because metal enhances this effect.