And he's at the beginning of the King of Eight:
And he's the voice of "Limbo" in this Scanimate film:
Thanks to a random commenter on YouTube, he's been identified! It's William "Rosko" Mercer, a prominent New York DJ. Here's more of that unmistakable voice:
All the above spots were done by Jim Henson, so Rosko must have been a favorite of his. Rosko's NYTimes obituary is here.