Anything useful in there? https://github.com/dapphp/securimage They seem to have audio support.
Topic on Talk:CAPTCHA
This system of captcha is somewhat equivalent to the present type of captcha in use, this wont provide any additional protection from the bots also the audio captcha in use can be made more convenient by improving Screen Reader as mentioned here: http://webinsight.cs.washington.edu/projects/audiocaptchas/