I would like to replace original quest complete sound from the one in World of Warcraft, does any1 know how can i acheive that?

Well normally, you'd learn the path of the sound as referenced by the game in Sound.bsa. Then, you'd make a sound file with the same name, put into Data/Sounds (sub-folders if applicable).

...but with the game's current state to not allow mod enabling (not without editing Fallout.ini, FalloutPrefs.ini, and something in AppData) - I don't know that there won't be any extra steps involved to make sure it is read. I DO know that textures require an additional step, for whatever reason.