Wife and I have bought a home and income property as our first house, having rented one in the past we thought sound travelling wouldn't really be too much of an issue. However the house we rented and the house we've bought must be made of quite different materials we are hearing much more than we did in the past.
Some googling around suggests this is a fairly common problem with many US based companies that specialise in helping to fix this. Problem is I'm not finding any NZ (Auckland) based ones. We do realise that nothing will fix the problem 100% and that it won't be very cheap, but keen to get a few quotes and assess our options.
Take a look at the GIb website to see great options around soundproofing I suggest. It wont be cheap but it also wont go away. PM me if you need any more help. I used to run a large commercial company that did sound proofing so have some knowledge.
Sound baffles/insulation will help reduce the noise a lot, sound proofing however is another matter entirely. Having been involved in many project containing both the aforementioned, I can tell you that there are many many options, what is most appropriate will depend on the specifics of a) the building, b) what you want to achieve (ie...how much noise reduction) and c) your budget!
We have just renovated our place in mt Eden had this done as well as new kitchen and bathrooms.
Best to get a builder in too look at this for your options.
Maybe as simple as or could be a nightmare .
Pm me if you want a good builder in Auckland anyway.