Analysis of audio captchas has led to a number of exploits for several captcha systems including recaptcha - which is what all that horrible obfuscation is trying to prevent.
The mission statement of the reCAPTCHA project is "Protect your website from spam and abuse while letting real people pass through with ease."
I don't think anybody is passing through the audio captchas with ease. Nor are they helping to digitize anything.
I think that the use of captcha's as a reverse turing test was always secondary for reCAPTCHA anyways. If someone can write software to solve the visual captcha's, that is a great accomplishment. Once that is accomplished, we will need a new type of reverse turing test. Perhaps we are approaching that point.
I'm interested to hear the perspective of users with a visual impairment on the audio captchas.