It seems there have been times when characters have been shown talking, but we don't really hear them talking (or can barely hear them). I wonder what went on.
During the end credits for The Muppets' Valentine Show, it looks like Rowlf is talking to Mia Farrow (we don't see his face when it looks like he's talking, but his body is moving as if he is), but we don't really hear him. Of course Jim Henson was performing Ernie in the scene, makes me wonder if he was going to do the voice live and forgot (or the microphone went out) while the performer operating him still moved him like he was talking (maybe, if it's a case of the microphone being faulty, he heard him from behind). Or maybe Henson was planning to dub some dialogue and the performer had Rowlf "mouth" dialogue that wasn't being heard.
At the end of the Paul Williams episode of The Muppet Show, when Sweetums and Thog appear, Thog moves his mouth like he's talking, but you can't really hear him. Though when the credits start to roll, I kinda hear his voice but can't hear what he's saying. Maybe the microphone wasn't working.
And in an episode of Sesame Street from the thirteenth season (I forget the episode number), in which David's grandmother sets up a vegetable stand on the street, there's a couple times when Bruno is talking to Oscar, but his voice is barely audible. It happens when there is a lot of noise going on, so I wonder if maybe it was being drowned out, though everyone else who talks can clearly be heard over the noise. I asked one of the Muppet Wiki contributors with a copy of the script about this, if the script says Bruno was supposed to not be heard talking, seems the script didn't call for his dialogue to be inaudible and that the script had him say "uh huh" (it looks like he's saying more, I kinda feel he's saying something like "I know, Oscar", though the script probably didn't account for adlibbing). I'd like to add this to the Bruno the Trashman page as an example of rare times he's talked but I'm not sure if I should.
I figured that the reason Bruno was usually silent was because Caroll Spinney had his hands busy with Oscar, but he's clearly moving his mouth a lot there. I don't know if the original plan was for him to be voice-less, but this was a year or two after he had brief dialogue in Here Comes the Puppets. Oscar's voice was heard just fine, so it's not like Spinney's microphone just wasn't working (and if by chance the sound was just dipping in and out, what are the odds that it'd be working for all of Oscar's dialogue while going out for both times Bruno was supposed to talk?).
Though one possibility hit my mind today. In most instances of Bruno talking, he just gets two syllables ("Put It in the Trash Can" is an exception, with him scatting and getting a full line, though the full line is off-screen). I wonder if it was hard to get the characters lip sync right, even in rare cases where his hands weren't busy with Oscar. And that scene shows him moving his mouth quite a bit (though the script gave him two-syllable dialogue). Could his voice being drowned out by noise (if that is the case, though the script doesn't seem to indicate it) have been a way to avoid poor lip sync? After all, if you don't really hear what he's saying, then how do you know the lip-sync is bad?