Oh gosh, when Dave Goelz was at the thing giving the thing (wait for it..it will come to me), he talked about how beeker worked. he also talked about how to do the voice, in that Richard Hunt didn't expel the noise, but rather did it when he was sucking air into his lungs instead.
Ok so for the mouth--i know he told us this one. And I fail. Sorry.
Tour of the Muppet Smithsonian exhibit (I knew it would)