I'm not sure how you can replicate Kefka's laugh with actual voices. I'm assuming the SE devs figured this out as well, given the lack of cackling in Dissidia.
I can hear it in my head, the way it should be. But I don't know how it would be handled. I don't know that any voice actor I can think of could do it on their own, meaning there might need to be some digital manipulation involved.