Keaton's voice sounded like that most of his life AFAIK -- less befuddled, but just as gravelly. If you ever get the chance to watch the National Film Board of Canada's Buster Keaton Rides Again, do so -- it's the making-of documentary for a short called The Railrodder that he shot in Canada a few years before he died, and it's amazing to see him at work.

ETA - it's viewable in parts on Youtube: http://www.youtube.com/watch?v=pygj0vOiIMo