Physicists have long been aware that when it comes to sound, inverse-waves cancel each other. -1 + 1 = 0. This has been applied to some fancy pairs of headphones that listen to outside sounds, and emit a reversed version in the speaker near your ear, with the result that you hear nothing. Useful in noisy places where one may wish to concentrate, or sleep. (Like an airplane.) But let me come up with another, more appreciative, use for this technology.
Many people love listening to really really loud music. It's bad for them, and the reason why one quarter the people my age or older have massive hearing loss. It also annoys everyone around them, who have to listen to their bass-lines for hours and hours and hours. I can't do anything about the deafening besides turning down the music, and they'd just turn it back up again. But I can use this technology to make their neighbours less annoyed.
Experimentation will result in a stereo system that has some speakers outside. Why? Inverse signal. You can turn the knob to 11 and your neighbours will hear nothing. It can do that because the sound is a known quantity (it does have access to the original signal), and the muffling that the walls and floor and ceiling makes are also known quantities. It can produce an exact reverse of what would be audible outside, thereby canceling it. The house is now perfectly quiet on the outside. Very loud on the inside, where your favorite music blares.
In fact, if you have very thick, sturdy floors, you could throw a party without your neighbors noticing at all. You'd be as quiet as it would be on an isolated estate, just the way your neighbours like it. (Warning: Will not protect against footsteps. That's why you need a thick floor.)