Hi Larian!
I would say, do your best, don't worry with the demo.
I think that the DK's voice is bad only in a few spots in the demo, especially in the first encounter with the DK where he explains the bounding.
Otherwise, I would say that short sentences he says in the demo are quite fine.
Example: "Not bad, human" (when you tie the rope in the cave): perfect tone of voice: it shows the attitude of the DK: he is my spiritual team mate, we both know it, so let's get together the best as we can. He's sarcastic and "friendly" in his manner. I liked it.
Another example: "I hate rats!" (the bone quest in the right middle cell): seems very expressive to me, while remaining calm in the same time.
Maybe a couple of hours recording new acting for those long narratives only would be enough, and fit in the time schedule without any need to touch the rest of the source code. The problem is not *that* bad, I think only some lines need re-recording. Not very expensive IMO (couple of days for 1 or 2 people), but could make anyway a difference in the first impression of games magazine testers.
Something I want to point to others: we did not test the full version, only the demo, where there is both good and bad in the voice, as explained above. But beta testers would have had plenty of opportunity to complain if it was staying that bad along the game, no? Larian programmers said in the forum that they were very surprized by the bad reactions on the voice in the demo. So they apparently did not have so many remarks from beta testers. Which is a good sign.
Good luck,
Philippe