Quote:
Thing is, in previous Blizzard games, the staff often provided voice work even in the finished product. It's likely they only bring in outside professionals for specific characters (e.g. Raynor, Kerrigan) or when they're unsatisfied with their own work. Consequently, I think most of the standard unit quotes we hear were made in-house.
It's mostly only Chris Metzen and Bill Roper who do (did) any voice acting for the games. (in a large capacity, anyway) Something which will likely only get more true (less employees) now that they've hired Andrea Romano.