If you set the site into "developer mode", you can't switch to the audio captcha when you click on "Verify using audio".
How to reproduce
1. Set up a vanilla drupal set and install & enable the mollom module
2. Don't set any keys but set the set the module into "developer mode"
3. Add the comment form for "article" and set the permission for anon to add comments
4. Add an article
5. Try to comment as anon on the article with "unsure" in the comment body
6. The CAPTCHA should be triggered. Now click on the "verify with audio captcha" link
7. Notice how nothing happens
If you go check the keys in the settings panels, you'll see that a set of test keys has been generated in the keys form. Yet, these aren't actually saved to the database to prevent accidentally overwriting production keys.