Peasy communicating through hundreds of Exes at once

The rustle of dry skin on metal, the endless clack of jaws, it all stopped. The exes froze in the sinking sunlight. Their collective arms dropped to their collective sides.
Derek straightened up and raised his rifle. “What the ****,” he murmured.
The eerie silence stretched over five seconds. Then ten.
“TONIGHT.”
Hundreds and hundreds of them spoke with one leathery voice that echoed across all thirty acres of the Mount. Some of it was clear. Some was just hissed air. Everyone understood it.
“TONIGHT THE SEVENTEENS ARE COMING TO KILL YOU ALL.”

Excerpt From Ex-Heroes, pg 359
Peter Clines
https://books.apple.com/us/book/ex-heroes/id585738407