I don't really do voices as a regular thing. I give examples to make an NPC stand out and occasionally pepper it in to maintain that sense of immersion, but definitely not constantly and not over the top. I take this approach both as a player and a DM. I think intonation and word choice do a lot of the necessary distinction between characters.
I will sometimes describe a voice, esp. if I cannot do it even as a brief example. When the party had an encounter with clockwork sphinx I described its voice as coming from deep within its body, "a scratchy cracking thing that emerges from its open mouth even as its lips do not move, nor does it seem to have a tongue. The voice is detached and distant and has a flat affect." And then, I added as an aside using a comparison the players would get even though their characters wouldn't, that is sounded like an haunted speak n' spell played through a gramophone.