Well they lived in Cyrodiil, they aren't going to sound like those from Morrowind. It's just regional. Besides, look at the real world, you can't see the deepness of voice is characteristic of even a family, let alone a race. Not to mention that the Dunmer were just the Chimer that were cursed to look dark. They are closely associated with the other elves so it would stand that they would also sound similar.
Yeah that's true in real life but for a game it needs to be different. It will help the game to have similar accents for each race. Use different actors but have them speak the same dialect so to speak.