In Season Five, Louis Chirillo has been replaced with Trevor Devall. And it is EXTREMELY noticeable, with the entire voice pitch changed and Trevor struggling to keep the same inflections with obvious throat pain (going by the rough, scratchy noises). And for all his effort he just ends up sounding like Hugh Test.
Mary Test was voiced by Brittney Wilson in Season One before Ashleigh Ball took over. For whatever reason, Brittney returned for Season Five before Ashleigh resumed the role again for Season 6.