Stichting BAM: Bescherming Akoestisch Milieu (Protection of the Acoustic Environment) for people who are highly sensitive to noise. Their goal is to fight "imposed noise".
They do activism and have information on quiet holiday resorts, and I would suggest make guides to quiet places to go out in the city. Most of the members are musicians, writers or journalists, and it turns out that Pamela Hemelrijk (libertarian journalist) and Paul Vitanyi (who wrote the Kolmogorov Complexity book) are among them.
A high-tech libertarian solution for this might be for each of us to adjust our own sensitivity... i.e. "standardize", but I think our technology isn't good enough to make wearing the appropriate equipment comfortable.
Another solution would be for sound producers to only broadcast to the intended recipients. Directional sound is a big step in the right direction. You-know-who-you-are: you've got your work cut out for you.
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What's the difference between
The Mute and noise-cancelling earphones? The Mute samples the undesired noise, but they can hardly assume that children screaming makes a constant sound.