There are several accessibility problems with this plugin.
First, note that the audio captcha option is the very last element in the form. Screen reader users have to navigate through the entire thing before they even find out an audio option is available, only after having to hear "click or touch the chair" and then a bunch of nondescript "internal links"... and then finally, an image called "audio image." Chances are, people will give up, before getting there, and when they do, they might not figure out that activating "audio image" will present them with an audio captcha. The audio captcha option should be the first focusable element in the captcha section (in the dom, if not visually), and should say "activate for audio captcha" or similar, instead of "audio image." The images for the visual options should have alt texts of empty spaces, so they aren't read at all.
But worse, when you finally do select the audio version, the audio captcha starts speaking immediately, BEFORE the screenreader is finished telling you that you clicked the button, so you don't even get to hear it!!! The info you need is drowned out by your own screen reader. There needs to be a delay, then instructions, then the code. Activating the "audio image" button again does NOT read you the audio captcha again, so you are completely out of luck.
I hope the developers can update this plugin, and test it with real screen reader users. It sounds like it is lovely for sighted users......