A "captcha" is ostensibly designed to weed out automated programs which are designed for getting past the security precautions of web sites. They usually say something like "type the captcha characters to prove that you are not a robot". Trouble is, the characters you're supposed to type are almost invariably presented in such a way that it's almost impossible to read one of the two words. To the folks who design these things, I want to scream, "Hey, STUPID, I am not a robot, but I am getting exceedingly angry when you force me over and over again to guess what your intentions were when you distorted the image so much that it was next to impossible to read.
And by the way, the audio versions aren't necessarily an improvement in terms of legibility.
Eventually one might be successful in getting past the captcha. But it can take seemingly forever, as it did just now for me when I tried to shorten a URL at a site run by Google.