What did you hear, which message did you get from the song now?
We are so used to hearing bleeps that our mind automatically fills in the blank now with what it expects to be there. This is just one of the many things that your brain will do for you without you needing to think about it. And, as we often point out here, your brain's conclusion about what should be there, is wrong.
Since this is Sesame Street, it leads to a quite humorous result.
(Via Richard Wiseman)