When working on the Knights of the Old Republic Campaign Guide, Star Wars author Abel G. Peña wrote a whole passage devoted to Nihilus' mask that was ultimately cut. It would have notably provided two scholarly explanations of Nihilus' speech. According to that cut passage, the Dark Lord's hissings may have been the first language ever spoken in the galaxy, or the raw dialect of the Force itself, untranslated by midi-chlorians.
So, Darth Nihilus was talking Forcesse by transmitting his intentions in the people's minds via Force visions?
__________________ The Ellimist: "Nadd is the 90 IQ guy who acts like a 70 IQ guy to make fun of the nerds"