The plugin Math Anti-spam consists of "a simple equation you must be able to solve in order to enter comments to a post. The equation is displayed as an image in a randomized color, font and position. An alternative to the image you can by clicking on the image, you download an audio mp3 clip that reads the equation for you". This audio clip is always the same voice, which is not randomly distorted or any other obfuscation method is applied.
http://www.securiteam.com/securitynews/5EP0C1FN5W.html
