Once at Kaminko's House, Prof. Kaminko will give you a Decoding Machine. With this if you answer a quiz correctly about the Muted Pokémon, it will restore their voices. With your decoder in hand, head to Gateon Port to un-mute the Pokémon there

Pokémon 1
So once in Gateon Port, head to the Sailor's House and you will find that his Oddish has gone mute. Talk to it and the Decoder will bring up a question. Below is the question and the correct answer

Q - What stone evolves this Pokémon’s evolved form into a beautiful Pokémon?
A - Sun Stone
Pokémon 2
Next head to the Old Man's House near the bridge, his Magnemite has gone mute. Talk to it and the Decoder will bring up a question. Below is the question and the correct answer

Q - What does this Pokémon evolve into?
A - Magneton
Pokémon 3
Next head to the second floor of the Krabby Pavillion and you will see two Krabby who both have lost their voice. Talk to the left one and the Decoder will bring up a question. Below is the question and the correct answer

Q - What is this Pokémon’s name?
A - Tarou
Pokémon 4
Now go talk to the one to the right and the Decoder will bring up a question. Below is the question and the correct answer

Q - What is this Pokémon’s name?
A - Hanako

After you have healed the final Krabby, it is time to head to Phenac City