Maybe I'm just going senile, but didn't the "experts" first say that they were 95% sure it was bin Laden's voice because the tape was of such good quality?
No, they didn't. The only specifics I've seen on why they concluded it was probably real is the story that claimed the US authorities played the tape to to Ramzi Binalshibh at Camp X-Ray, and he said, "Yip, that's Osama." But, in the words of a famous British call girl, "He would say that, wouldn't he?"