I decided to address the problem of the location of the captcha code on the input and moved it to be the last field on the last step of the registration form.
Doing so has solved the problem regarding valid input being rejected when a new capcha code is presented. It has not solved the question of a disparity between the audio and the visual when a new capcha code is generated after a captcha error - the audio value is always the capcha code previously displayed.
My immediate problem has been resolved and I thank you sincerely for the significant time and trouble you have taken in the investigation.
If I do have any caching enabled I have been unable to find it in Joomla!, CB or CBSubs, so maybe I've missed something. I doubt that any of our users make use of the audio function - I just stumbled on the disparity in the course of testing.
Thank you again.