The Volume of Silence. Shadow World Paradox, #6

In the city of Echo, your social status is determined by your "Sonic-Score"-the more people talk about you, the louder your personal frequency, and the better your life is. 13-year-old Kael is a "Mute, " a lowest-class citizen who is literally fading from view because no one mentions his name. While hiding in the city's plumbing, he discovers an underground society of kids who have learned to "store" silence in physical canisters.
They aren't trying to be famous; they are using the stored silence to create "Void Zones" where the government's tracking frequencies don't work.