here's a tutorial for those who want to change from english to korean voice.
1. Make a backup of the "Sound" folder located at your Grand Chase folder. Don't forget this.
2. Download these files:
3. Copy and Paste the downloaded files to the original "Sound" folder.
open your original Grand Chase icon. Use the "bypass GC launcher" posted in the sticky thread, or else GC will download the english version.
In case it's tuesday, "the patching day", you may backup only the three files, the Korean voice and put back the original(recently backup) "Sound" folder. After the patch, you may backup again the original folder(if it's updated) and copy and paste again the files.
here's the website where i got the files. it also contains other sound files but i don't know what it is =P