Can't say I have. Is it not as one might imagine?

It's difficult to describe. Go sign up for a new Google account [1] and click the wheelchair symbol to activate the audio captcha.

[1]: https://www.google.com/accounts/NewAccount

On the off chance someone reads this, I'll try to describe it: over the top of random, unintelligible babbling, a voice reads alphanumeric characters. Parts of it are about as difficult to understand as some captchas are to read recently.