Capsule 1: The first Pokémon to be cured from their mute ailment would be the Old Lady's Combusken inside her house in Agate. Talk to the Combusken and choose to use the Voice Capsule and it's voice shall be restored. After that, go back to Kaminko's Lab and get the next Voice Capsule
Capsule 2: The next Pokémon to be cured is the Poochyena in Agate Village. Talk to the Poochyena and choose to use the Voice Capsule and it's voice shall be restored. After that, go back to Kaminko's Lab and get the next Voice Capsule
Capsule 3: The third Pokémon to be cured is a Taillow, again in Agate Village. Talk to the Taillow and choose to use the Voice Capsule and it's voice shall be restored. After that, go back to Kaminko's Lab and get the next Voice Capsule
Capsule 4: The next Voice Capsule is to heal the Wobbuffet in Bilbo's house in Agate Village. Talk to Wobbuffet and use the Voice Capsule...however Capsule 4 will not work on Wobbuffet for some reason, so you need to head back to Kaminko's House
Capsule 5: Once you get there, Nett's Sister will E-mail you from Pyrite saying that her Shroomish has also gone mute, Kaminko will give you Voice Capsule 5 so go to the ONBS station in Pyrite City. As usual go to her Shroomish, talk to it and use the Voice Capsule and her Shroomish will be healed. After healing her Shroomish, Nett's Sister will tell you that there are reports of Pokémon going Mute in Gateon Port & Phenac City. So go back to Kaminko's House.