Well, Gonzo is definitely the biggest example there. As one of the few Muppets who's had the same performer from Day One, (Save for TGSCS which doesn't really count because that was sort of a proto-Gonzo there.) it's been pretty evident that Dave has really developed the voice over time. Just look at the Gonzo voice comparison section on You Tube.
I've actually noticed that Big Bird's voice has changed a lot over the years as Caroll Spinney has aged, his voice is deeper now.
Yeah. I've seen some of the 90s episodes with Big Bird on You Tube and even then the voice was deeper than it was in the 70s and the 80s.
Janice went from being a valley girl from TMS Season 2 to GMC, then starting with MTM, I felt her voice started to get deeper, and more out of character to the point where her character was really not much a valleyy girl,
I'm not sure I agree with that. I thought Richard's valley girl Janice has been pretty consistent. (Save for the Don Knotts episode before Richard had the voice established.) But oh well, everyone is entitled to their opinion.
I loved raspy voiced Elmo. Seems like that was lost right after the "Tickle Me Elmo" doll was released. It really sounded like Elmo was a "monster child". Nowadays it sounds like "a child that just so happens to be a monster".
I was wondering about that since I knew Kevin's Elmo voice has changed over time, but I was never sure exactly what point his voice changed. I notice even in the early 90s, Kevin seemed to use the raspier Elmo type voice, so you may have hit the nail on the head by mentioning the time TME was released. I guess Kevin was going for a cuter monster sound that would work for the voicing the toy versions of Elmo.