The pitch shifted voices are by now a hallmark of the series, and do conviently mask the fact that Lanning does nearly all, if not all, of the voice acting in the games (as jiki said). I dig them quite a bit, it's easily some of my favorite western voice work.
There also seems to be something quite significant about the homogeneity of all of the different races in the Oddworld -- all looking alike, all speaking alike. There's a dynamic there in all the games about the invidivual vs. the masses -- Abe rising above the other Mudokens to the level of hero, Munch being the last of the Gabbits, and now this loner Steef -- that I think goes a little deeper than the economy of reusing character models and not having to hire different actors for every character.
That's just sort of an off the cuff too tired to think response, but I really do think a lot of it is intentional characterization (and communist manifesto) and not laziness. I'm not sure exactly when this happened, because I was never that fanatically into the first two games apart from appreciating the puzzle-y gameplay, but somewhere a couple hours into Munch I suddenly completely bought into Lanning's vision and universe. Something just totally clicked.
Favorite moments of the demo:
"Now look, Stranger, we don't want no problems now, you just settle down aight?"
the way the enemies REALLY REALLY hate the chipmunks and can't help themselves from needing to stomp on them
the chipmunk calling you 'professor'
