Captcha is the easiest way to stop spammer software logging into your Web site and adding comments with links to unacceptable sites.
Download Captcha from drupal.org/project/captcha. Version 6.x-2.1, from 2010-Jan-02, is a 161.2 KB download.
Captcha includes Image Captcha but does not have an audio play option for the image. See drupal.org/node/126812 for possible progress toward an audio play component.