“Feilfaux” isn’t a common English word, so there may not be one agreed-upon pronunciation.
If it’s a name, one possible way to say it is “FAYL-foh” (rhymes roughly with “pail” + “foe”). The ending faux is often pronounced “foh.”
If you share where you saw the word, or the person’s name or language, I can help find a more certain pronunciation.