Are you sure you want to go the captcha route? Why not ask for an email address and email them the access code? Emailing the access code eliminates problems for blind users because we can assume they have a way to read/hear their email. Plus, you then have a way to track users and contact them. If you want your service to be anonymous, then that might be a bad thing.

On the other hand, bots could generate a million fake accounts by using a million self-generated email addresses, but it that really what you're worried about? Is your registration for a service that bots would want? Does it include giving users an email address or something?

Hope that helps!
--Pileofrogs