This would be a great addition. NPCs as they are are far too silent. A voice, whether it be a mumble, grunt, or greeting would give them much needed personality. As mentioned, something along the lines of BG2, or NWN would be fantastic.
Another note... This is just an idle fancy, likely far out of the question. Combat taunts. One of my favourite aspects of many RPGs. They often are the most memorable parts of games, such as Morrowinds and Oblivions taunts shouted by an overconfident bandit before death. I would love for this game to include such taunts, but that is just me.