The Simple Machine’s Forum audio Captcha that has been hardened from attack. I have contacted SMF about this flaw and it has been verified.
I go into greater detail of how i am able to break this captcha here: http://www.rooksecurity.com/blog/?p=6
Exploit Code: http://www.rooksecurity.com/exploits/smf_captcha.zip
This captcha has been broken before and exploit code is available here: http://www.securityfocus.com/archive/1/471641 . The fix was to add a randomly generated static.
I was able to write code to decipher the audio file using a Fuzzy Logic comparison http://en.wikipedia.org/wiki/Fuzzy_logic . The comparison is the Hamming Distance http://en.wikipedia.org/wiki/Hamming_distance between the original audio file and the damaged one generated by the Captcha.