You can hear Jim's and Frank's voices among Doc's goons during the scene where they're being attacked by Miss Piggy. But that was dubbed in post. They're not actually in the scene. I can't spot Jim anywhere in the film, nor can I spot Jerry or Richard. Steve Whitmire can be spotted as an extra in the crowd shot where Miss Piggy first lays eyes on Kermit. He's standing right behind Kermit and Fozzie. Also, during the end credits, when Nigel, Boppity (Blue Frackle), Marvin Suggs, Rowlf, and Beaker are talking to each other about how great the movie was, it sounds like Dave Goelz is doing Nigel and Jerry Nelson is doing Boppity.