Crying Baby Sound Cancellation
Advancing technology has continued to improve the quality of our lives, and one day, it will hopefully silence all the noise. Give it a few years, and a product like this will be readily available (because the idea is the hard part, not the technology):
(and yes, I know my fake advert from the future isn't politically correct, as most everyone dislikes a crying baby, not just male father's who work from home)
Active noise cancellation works by inverting the sound so that the original + the inverse = zero. This results in silence, in theory; or a reduction, in practice. It works well in small spaces, so headphones are perfect. You might have seen such things for muting the omnipresent engine hum inside commercial airliners.
But when you try to use it on a larger scale, the sounds reverberate off their surroundings in all directions. When the inverse bounces off different surfaces at different distances from the listeners, there will be different phases of the wave reaching the listener at various locations.
Now you have the original + (inverse, but out of phase) does not = zero.
The result would be either a sound that seems to change volumes, periodically; or just louder than the original.
But that's today; give it two or three decades and all these things (like amplification compensation levels) will be miniaturized and adaptive enough to overcome such irregularities.
…All too late for me, though.