Almost all the Dungeon Masters I have played with have done voices for unusual races but I've found that goblins and kobolds end up sounding similar and once I got the two confused. So far only one Dungeon Master has done dialects of human languages and did a very convincing job.
Let's see, 2 for Dungeons & Dragons fourth edition encounters, one from Warhammer fantasy, one from Dark Heresy (Mostly the orcs in the warhammer universes), 3 with Pathfinder society organised play at conventions, and one from a Pathfinder home game. So all 7 Game masters I've had did voices even if I only played a single session with them.
So, I can see doing this for a critical exchange. If the Dungeon Master does this sparingly it would to the game I think. If the Dungeon Master does it all the time it would become a distraction and annoying as many of you said.
Edited: Abnninja on 25th Sep, 2017 - 11:35am
I don't really like voice overs. Most Dungeon Masters can't do them well and it takes away from the scene. That is a perk to play by post games. You use your imagination instead of having to pretend that your Dungeon Master using a horrible orc voice is actually the orc Npc.
Name: Steel
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Comments: You don't even have to do a voice over, just imagine a Dungeon Master with a really squeaky or whiny voice trying to describe things. Its like hearing chalk screech on a blackboard (For those of you who remember those).