I can only think of three times when Bruno was most likely performed by somebody other than caroll Spinney.
1. In the scene in Follow That Bird where everybody is preparing to leave. Bruno is in the crowd as Oscar is driving, and Oscar has a speakign part in that scene. I doubt that they would have had somebody else perform Oscar and then have Caroll Spinney dub the voice later just so he could perform a non-speaking role.
2. I once saw an episode of Sesame Street that ended with bruno carrying Irvine in a trash can. I think Irvine started day care in that episode. Anyway, in that scene with bruno transporting irvine, Oscar was also in a trash can in the regular spot that his can is. Martin P. Robinson performed Irvine, so he was most likely performing bruno in that scene.
3. In The Muppets: A Celebration of Thirty years, Bruno was in the audience. At the beginning the camera moves around the audience, and Big Bird, oscar, and Bruno all appear in the same take in different areas. I think Caroll Spinney was probably performing Big Bird in that scene, although none of those characters spoke at the beginning. During the closing song there was a shot where Bruno was seen and Big Bird and Oscar weren't, but I don't think Spinney was performing Bruno in that scene. I don't think they swithced performers for characters during the closing number. It's been awhile since I last saw this special, but I think Brunos voice can be heard in the chorus of that scene.