At some point in the 1980s or 90s they replaced the overly enunciated voice acting with more natural sounding voices. To me part of the fun of Gumby is laughing at it as well as being amazed by it. I love the way the voices sound in this video.
It used to be that the most common way to make a recorded voice higher-pitched (or maybe the only way at the time, I'm not clear on this) was to speed up the tape. So you had to speak super slowly and enunciate, or the end result could end up sounding like an old casette being played while on fast-forward.