Yeah, so far I've only heard Richard's Piggy in the Juliet Prowse and Rita Moreno eps. It is a bit strange hearing her sound like that. I sort of consider the voice more like Richard's Janice without the valley girl accent. But I do hear Gladys in there, or even Don Music. No offense to the people who wrote the guides online, but they don't always clarify who's performing Piggy in what scene, so I look forward to getting the DVD so I can see which Piggy is Frank's and which is Richards. Especially in the opening joke in the Kaye Ballard ep. It seems like they were giving Piggy to Frank by that time, but I think it could have been Richard performing her while Frank was performing Fozzie. I've been thinking a lot about that one.
I also find it interesting that I read that the reason they had Richard's Piggy sing is because they thought he was more vocally talented for that. However, I think Frank's vocally talented too. After all, he was the one who had Bert do the pigeon and said "C is for Cookie." Not to mention some later songs by Piggy. (Ex: "Never Before, Never Again," "Trees," "Pig Calypaso") Still according to what I read it does seem like they were going to give Piggy to Richard full-time. I wonder what made the writers change their minds.