From Bulbapedia, the community-driven Pokémon encyclopedia.
Cacophony (Japanese: そうおん Noise) is an Ability introduced in Generation III (unused and thus dropped in Generation IV), that avoids sound moves. This Ability is identical to Soundproof, but only usable in Generation III games.
Cacophony is an Ability coded in the Generation III games, but no Pokémon have it. It cannot be acquired by normal means, hence it was removed in Generation IV.
Due to its use on the Exploud released in Great Encounters, it is possible that this Ability was initially intended to be that of the Whismur family. The reason for the change is unknown; however, 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.