I have some experience in this field, having worked with adaptive technology for a few years. But, due to the topic of this forum I wonder about your intent?
Are you going to be using it to create "audio" captcha you can script? or do you want it for people to be able to read your blog?
Anyone remember the old Apple Text to Speech program, simply called speak?
Chances are you don't have a mac so lets look at other options:
The industry standard (for disabled assistance) is JAWS or Zoomtext.
Both of these products are designed for people with low to no vision. Starting at $700 and up at last check.
As a more resonably priced option check out: