For each CAPTCHA (anti-spam system) that is either just sound or visual, an alternative must be provided. For example:
A sound alternative for a visual CAPTCHA.
A visual alternative for a sound CAPTCHA.
A text alternative in the form of a simple question for a sound or visual CAPTCHA.
Etc.
Warning
CAPTCHA systems are designed to block spammers and use increasingly sophisticated methods to obstruct them. CAPTCHAs are therefore more and more difficult to decipher. By definition, a CAPTCHA will therefore never be completely accessible. With respect to accessibility, the best solution is to avoid them.