When working on the Knights of the Old Republic Campaign Guide, Star Wars author Abel G. Peņa 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?
__________________ Azronger: "Nadd is not a Sheevite. He hates Sheev. Avoid him."
__________________ "I judged the Poles by their enemies. And I found it was an almost unfailing truth that their enemies were the enemies of magnanimity and manhood. If a man loved slavery, if he loved usury, if he loved terrorism and all the trampled mire of materialistic politics, I have always found that he added to these affections the passion of a hatred of Poland. She could be judged in the light of that hatred; and the judgment has proved to be right."