Not really that rare, seeing as it just requires an injury to your hearing to acquire, like Baseball Boy. TBH it feels rather flimsy: wouldn’t elderly members of the village lose their hearing and suddenly be able to see them? Or perhaps, because of the frequency, you’d have to have a very SPECIFIC type of deafness, or total deafness to truly block it out. Hence the earphones the scientists use are able to still hear people’s voices, but the mind altering frequency is blocked.Ironic that Parasitus Coatlis is nothing like the benevolent Coatl it is named after, other than being a winged serpent. The few people who cannot hear its resonance are the only ones immune, likely a rare genetic thing.