CAPTCHA and Visually Impaired Users

I had a visually impaired user contact me saying they could not sign up on the forum since they can't read the CAPTCHA confirmation image. I have it enabled to keep mass automated registrations from occurring as many forum owners do. However, is there a way to make the image data generated where someone could click on an audio file and hear an audio version of this item? To prevent automated software from decoding such a file, background noise can be added to the audio file so that it would confuse that kind of software but not a human who would be hearing it. Thanks for your time and consideration.