Reproducible with the following setup:
1. Use secure pages module to secure some of your paths
2. Now to avoid mixed content warnings, you should set your /mollom/* paths to use https too for the benefit of those pages.
3. However, if you still have a form mollom enabled on a non-https page, when switching to audio, the client code only gets a redirect (which it does not handle).
Result: audio captcha will not replace the image captcha, despite repeated cycling clicks on the image/audio captcha links.