From Bulbapedia, the community-driven Pokémon encyclopedia.
Cacophony (Japanese: そうおん Noise) is an unused Ability introduced in Generation III. Its effect is identical to Soundproof: it negates the effects of sound-based moves on the user with this Ability. It exists only in Generation III games, having been removed in Generation IV. No Pokémon have it, and it cannot be acquired by normal means.
Due to its use on the Exploud released in Great Encounters, it is possible that this Ability was initially intended to be related to the Whismur family. The reason for the change is unknown, given several other Abilities are equivalent in effect to another (such as Air Lock and Cloud Nine).
Pokémon with the Ability Cacophony would be immune to the following moves.
Pokémon with Cacophony
No Pokémon have Cacophony as their only Ability.
No Pokémon have Cacophony as one of their two Abilities.