I gave some thought to an audio capthca... Basically some unwitting fool would have to record A-Z 0-9 in wav's and then we'd have some sort of code pull a random set of the images and create links for them and have it so a user would have to click on each section: [1] [2] [3] [4]
Then have it so an alt states "Audio CAPTCHA Character #"
I donno, was just an idea, and I wasn't going to say anything till i threw together a small "proof of concept"... but i'm busy tonight lol