A good idea patryk, could mean some nice extra traffic. I guess for people with screen readers you just use the alt tag instead of having to build an audio library for every captcha.
UPDATE: Just finished off the coding for it now: http://www.bellsdomestics.co.uk/blog.php
All working now so it's time for some content, then solve Google+1 and Facebook button issues and finally W3C compliance.
Other than that and interestingly I think a robot or someone messing around who has seen this thread put a spam comment in after I got recaptcha to work!! I really do hope it is a joke and not a sign of things to come because otherwise, what is the point of having a recaptcha?