For the one living above you....
First you need to investigate and make sure it is not structural
noise from the building itself.........some buildings have internal
noise from the pipes......
Go up and take a closer listening at night......make sure it is
human activity.....then if you can pinpoint the place, why not just
knock on the door and talk to the resident......ask nicely to keep
the noise down.....simples.
Second,
For the resident opposite you who like to play music
loudly....same remedy, go pinpoint the source....knock and
talk.....
For teenagers...again same remedy.
It is important that you try to be nice and polite, and make a
request that they keep the noise down.....
If you write a letter, lodge a complaint...etc.....it wont be
effective....the most effective thing to do is face-to-face
communication directly.
If you really wanna solve the problem, you have to do this.