Last night I saw the street scenes from episode 1880, noticed a brief speaking appearance by Bruno, saying "okay". And I noticed in both that and the 1980 World Puppetry Appearance, he talks a little softly. There's also a season 13 episode where he's shown talking but can't really be heard (there is a bit of noise going on there).
It makes me wonder if originally, when Bruno talked, he was supposed to talk real quietly. In Follow That Bird and "Put It in the Trash Can" he can be heard a lot better/louder than in those. Muppet Wiki mentions two other episodes where he has dialogue, I wonder how well he's heard in those.