Well, possibly not the best analogy since it was Andy Serkis who did all the motion capture for Gollum, but I know what you mean! The thing about the voices is, yes, you can get people to immitate them, but they rarely have to actually act. Could you get people who sound like the cast? Probably. Could you get people to provide the same intensity and emotion as the cast? I'm not so sure. Joss would want the best, and if he can't get the original cast or a set of people who can provide an equally good performance, he wouldn't do it, and I'm not so sure I'd want it.