For other curious people:
Download: DN JP Voice Pack - Dragon Nest Source
This is the DN JP voice pack if you can't stand the NA voices. It contains NPC and character voices. You don't have to use all of em. You can choose from one or both. Enjoy..
PS: I like the NA voices so I won't be using this. This is an old download, but I used it long time ago and it works fine.
Download Link: DN_JP_Sounds_20100821.zip
Just extract whatever contents you want to have (You may need to extract it to desktop or other place first) into your main Dragon Nest folder next to your other paks. That's all you need to do. Client will start up and use JP voices.
Give it a try and enjoy.
*Contents will be a .pak file.*
Contains: Cutscene Pak, Character Voice Pak, NPC Pak.