The style that I am shooting for can be heard on this video: http://www.youtube.com/watch?v=nZlPiehYpVM at 2:18 ("Blades of Steel")
I have tried playing with equalization, hi-pass/low-pass, and various filters, but I can't seem to get anywhere close to this effect, where the "s" sounds come off as static, and the annunciation of consonants is muted (possibly an attack filter?). I can't find any documentation through Google search, either.
I know very little about the sound architecture of the NES, other than that music was composed with Square and Triangle waves, and the "samples" were presented through a separate sound channel. On NES sound emulators I use, this channel is usually labeled "PCM," (Pulse Code Modulation?) The NES system itself was 8-Bit, but I do not know if its sound system was.