According to a couple of Tweets by Overhaul Games producer/writer Phillip Daigle (here and here), the writing in BioWare's Baldur's Gate II (and presumably Throne of Bhaal) included approximately 1.2 million words of text, but the forthcoming Enhanced Edition will sport an extra 350,000 words. And some of that will be voiced:
#BGEE #BG2EE For all of the expanded content in BG2EE it looks like we've written 350,000 words. And now we're trying to voice some of it. I'm not sure the game needed to get even bigger than it already is, but I'm certainly not going to complain about extra content.
#BG2EE #BGEE for reference Gaider (one of the original writers) said BG2 is around 1.2 million words. So we'll be at nearly 1.5 million.