Cacophony, whose Japanese name translates to Noise, was an ability introduced in Generation III, but was ultimately removed. This ability was intended to be a counterpart, if not the original version of, Soundproof.
What are the differences between Soundproof and Cacophony?
The fundamental difference between Cacophony and Soundproof is the ideas behind each ability.
Cacophony manages to block Sound based moves due to loud noise generated by its user, which is how Whismur-line blocks sound:
Sapphire: Loudred shouts while stamping its feet. After it finishes shouting, this Pokémon becomes incapable of hearing anything for a while. This is considered to be a weak point.
However, all the other species that get Soundproof either have massive armor that blocks sound(Shieldon-line) or are incapable of hearing because they lack ears(Mr. Mime, Electrode etc.)
This creates a logical contradiction, as Exploud-line is perfectly capable of communication with others:
Sapphire Exploud communicates its feelings to others by emitting whistle-like sounds from the tubes of its body. This Pokémon only raises its voice when it is in battle.
which doesn't make sense if it is incapable of hearing.
While it is true that Whismur-line seems to have a biological mechanism for blocking out sounds, other Sound based species who don't have such a mechanism are rendered incapable of blocking Sounds.
What advantage would Cacophony bring?
Cacophony, if it is brought back, could be obtained by Sound-based Pokémon, such as Chatot, Jigglypuff, Kricketune, Woobat, as a means of blocking Sound based moves.
Conversely, Soundproof could evolve into an ability that includes other methods of blocking sounds.
Perhaps more interestingly, if Cacophony is brought back, it could be obtained by certain moves similar to how Worry Seed induces Insomnia. For example, Uproar can temporarily transform its user's ability to Cacophony.
Is it likely that this will happen?
Probably not, since the current repertoire of Sound-based moves are either weak, exclusively Normal-type or rare.