To be honest i wonder if a good captcha can be solved automatically, i have seen this video about XRB, but the weakness was the unlimited number of try. I'm not sure a bot could solve a capcha at the first time, especially if it's a dynamic one (Once with image, once song, once number) for each fail you wait 1 hour.

However i have to say i really like the approach you propose. It's a very good idea.