From Glitch City Laboratories
CACOPHONY is an unused Ability which is only known to exist within Pokémon Ruby, Pokémon Sapphire and Pokémon Emerald. The ability was removed as of Generation IV.
This Ability is identical to Soundproof, in the sense that any Pokémon with CACOPHONY as an ability will be immune to attacks such as Uproar and Grasswhistle, however no Pokémon can ever have CACOPHONY as an ability without the use of a game-altering device. Since this ability is never mentioned within the game, the term "CACOPHONY" is normally never seen through normal gameplay.
When a Pokémon with the CACOPHONY ability is transported to Pal Park an error trap will revert the Pokémon back into having the ability which it should usually have. For example, if a Ninjask with the CACOPHONY Ability is transported to Pal Park the game will automatically give the same Ninjask the Speed Boost ability.
It may be of note that on a Japanese version of an Exploud card one of Exploud's moves is Cacophony, however when those cards were officially translated into English this was changed to 'Ambient Voice.'
- Serebii.net's list of abilities in Generation III (note that "Cacophony" is listed)