Korean Voices:
You dont need to download anything. The Korean voices are already included in your aion folder.
If you have the Japanese voice pack installed, then you need to remove it first. Make a backup of it just in case you want it back. Go into Aion\L10N\ENU and delete the Sounds folder or Sounds.pak file.
1.
Make a new folder named Sounds in Aion\L10N\ENU
2.
Copy the voice folder from Aion\Sounds into Aion\L10N\ENU\Sounds
You should end up with Aion\L10N\ENU\Sounds\voice
3.
Done. Play the game and enjoy!
This will let you have the original Korean combat voices (the ones that were used in NA Closed Beta) while the NPC and Cut Scene voice overs will still be in English.