Securing Web Forms with PEAR's Text_CAPTCHA
Subject:   Visually impaired people
Date:   2008-06-13 23:37:23
From:   miguel.garcia
Response to: Visually impaired people

In the case of Visually impaired people many websites use sound for validation, so, the user must type what he hear.

I think you are right with the race condition, a possible solution is to use a .php file as the source of the image and in that file generate the image with the Text_Captcha and echo it.