Keyes explained why we only let natural born Citizens be president and Barry said he was running for Senate.
It did happen. I will not bend over for the Obots.
I think what happened is that your recollection of the debate was colored by the eligibility controversy that followed. So far, there's been no evidence that it happened the way you described.
I also remember people later not being able to access that same audio link soon after I heard it.
How do I explain Keyes' denial of said audio? I don't, but neither will I deny that I heard the audio and heard ObaMahdi say what he said.