Michele Diodati is one of the most influent accessibility experts in Italy. He developed a very simple technique to make a CAPTCHA accessible. This technique relies on the existing difference between a human being and a robot. Basically, a robot is unable to understand the meaning of a human sentence or to answer to a simple question, such as "What is the capital of Italy?". Known that, you can tell robots and humans apart by simply asking them a question. If they answer correctly, the form will be submitted, otherwise not. I've tested this kind of filter on a recent project and it works great! No incoming spam!
What's more, you don't need to use an audio file to address the problems of persons who can't see the image, because all they have to do is to insert some text in a text field. That's all. I'd like to see more sites implementing this approach. In the meantime, please thank Michele for his idea.