I don't mean to be a pessimist, but I don't think we are going to find 50-some good voice actors for the leaders in Civ4 if a similar set was created for Civ5. I'll stick with Leonard Nemoy for the tech discoveries, and just unique animated leaderheads otherwise.
How about all those voice commands in different languages for units? That's 32 civs with about 16 commands I believe. In all cases most of those 50-some leaders are already dead so you don't reall need a good voice actor. With the new Star Trek movie coming out, I want to hear Leonard Nemoy and Patrick Stewart.