The hardest captcha I've come across is the audio one used by Google on https://www.google.com/accounts/NewAccount . Click the little wheelchair icon to hear it. It's so hard to decipher, it almost comes across as a cruel joke at the expense of blind people. These are much easier, in that I can actually understand them.
Once at MIT a blind student was showing me his screen reader. I couldn't make sense of the sounds at all, and asked if it was in some special code. He said no, it was normal language, just really fast. He had it read the current line on his terminal (this was late '80s). I could see the words it was supposedly reading, but I still couldn't map them to the brief burst of sound that came from the screen reader. So mayb…
Yup, my sister is blind and it's the same thing. I hear her having something read off her screen and it's just way to fast to even notice a single word. I asked her about it once and she told me that she doesn't even have it set to read as fast as most of her friends!