The Nagina in particular probably have a hissing quality to their voice perhaps "typical" of snake-like creatures, but it's more of an undertone. Think the sub-vocals of like, turians from Mass Effect or something, where it's almost like a faint reverb underlying their speech. Talking to them probably gives off the sensation of speaking to someone who isn't actually speaking the same language as you, but you understand it anyway.

Most of the demons probably have an effect like that in their voices, it just differs based on the species. The Nagina sound snake-like, the Barghest have a wolfish growl, etc. The Hecubi and Hellions, as the more "humanoid" of them all, are very different. Hecubi voices are pretty "normal" and dialects/accents are more based on regional habits/personal tendencies. Hellions have very deep, harsh voices in general.