Speaking of Zoot singing along but not getting any of his own lines, I wonder if Dave Goelz normally sang in Zoot's voice whenever Zoot sang along with other characters but couldn't be heard. I also wonder if Steve Whitmire sang in Lips' voice in such scenes where Lips sang along with the others at the same time, and couldn't really be heard. I think that his voice is a bit simialr to that of Rowlf and Dr. Teeth, and most scenes where he's with a group of characters singing feature at least one of those two, so it would be hard to make out his voice.
In The Great Muppet Caper, when Kermit and Miss Piggy get on the Happiness Hotel bus, there's a Steve Whitmire voice hear doff-screen, saying "Let's hit the road! How about a little traveling msuic!" I thought this was Lips, and I could still be right, but there it sounds different from in Boogie in the Barnyard. Of course, in that scene he was speaking, and in Boogie in the Barnyard he was singing (and at the end, when he says "and me", it's a bit quick and brief to really pinpoint what he sounded like). Or it could have been Steve Whitmire attmepting to give him a less african-sounding voice (when he wasn't even seen on-camera). But in this scene, he soudns a bit like the turkey from A Mupet Family Christmas. I assume that it wasn't one of the rats or chickens talkign, and it obviously wasn't Rizzo (and I don't think Rizzo was even in that scene).