Sound CAPTCHA doesn't work on the registration form

Preconditions: Have the JCaptcha activated with the Sound option, from Administration (with the tick on "Require CAPTCHA to register" from Users & Rights > Registration).

Steps to reproduce:

1. Access the wiki as an unregistered user
2. Click on Drawer > Register

Expected results: The guest sees a form where he can register and validate with a sound CAPTCHA.

Actual results: The sound CAPTCHA doesn't work on the registration form.

NOTE: This happens on Edge, IE, Chrome and FF

It is working in the Comments section.