I found three great clips, all with Big Bird and Snuffy.
One of them had Big Bird and Snuffy imagining that they were at a disco. I wonder if that was Jerry Nelson or Michael Earl Davis performing Snuffy... it definatley wasn't Robinson. It sounded like Jerry Nelson's Snuffy, but I also feel like he sounded slightly different. If it was Davis, then he did a good job with Snuffy's voice... but it is possible that Nelson's Snuffy started to sound a bit different in his last years as the character (as many characters voices have changed a bit over theyears regardless of whether they kept the same performers).
And there were two clips that I remembered seeing asa child, but I thought that these were the same skit. One of them was the one where Big Bird and Snuffy jumped on Big Bird's trampoline. The other one was If I Was a Cloud in the Sky. Somehow I always thought that the trampoline skit lead to the scene where Snuffy was imagining himself as a cloud, singing the song. But in these clips, they are clearly different... Snuffy just jumps onto the roof instead of into the sky, and the other sketch begins and ends with Snuffy in Big Bird's nest area, dressed as a cloud.
I wonder if either of those last two sketches were made before or after the adults saw Snuffy. The Count did clearly see Snuffy in the trampoline skit, without saying anything about him being imaginary. But when The Count saw Snuffy, he acted as if he had never seen him before. I know that some Mupepts saw Snuffy before the human adults did. But maybe it was the first time that The Count saw Snuffy, and maybe it was broadcast after the adults saw him (The Count wasn't there when the adults saw him... and very few Muppets were in that scene).
If I Was a Cloud in the Sky is more likely to have been before Snuffy was seen by the adults. Snuffy only appears with Big Bird, the adults don't enter the scene, Snuffy doesn't leave so that a human can appear and miss him, and there is no mention of whetehr the others can see him.