If you are playing the English version, the game requires the English pair to be present, intact, and matching.
: If you want to play with audio in another language (e.g., French) but the in-game menu doesn't allow it, players often "disguise" other language files as the English ones. For example, renaming sound_french.fat/dat to sound_english.fat/dat forces the game to load the French audio assets.
Without the .fat file, the game engine wouldn't know how to navigate the .dat file, rendering the audio data useless. Modding and Extraction