UPDATED: It seems really easy for Chicago local media, especially, to determine whether or not this is the right tape of Michelle Obama. All they need to do is pull the tape from this 2004 Conference (particularly the panel Michelle was on), which should be readily available (since this was a very large multi-day event). All a journalist needs to do is watch this tape: if this is not the right one, they'd know immediately and this issue would be debunked.
Actually, what makes more sense to me is that if someone actually has a tape with her making a rant, they provide it. Then the issue would be confirmed. It's ridiculous to suggest soemone else go debunk an unconfirmed rumor by finding examples of tapes that are NOT the alleged rant!

It's as if I claimed I had a tape of me being abducted by aliens, and I said if you think it's not true, prove it by finding tapes of me NOT being abducted by aliens.

Debunker: Well, here's one of you at your high school graduation not being abducted.

Me: No. That not the one I'm taliing about!

Debunker: Here's one of you at a picnic not being abducted.

Me: Nope! Try again!

Debunker: Here's one of you at Disneyworld not being abducted

Me: Sorry guys, those aren't the tapes of me being abducted that I'm talking about!

It doesn't make any sense. You don't prove something doesn't exist. If you are going to claim something DOES exist, then you prove THAT.