If your site uses multimedia presentations, such as applets, images, sounds, or video clips, it needs to have textual equivalents wherever possible.
You should provide captions or transcripts of important audio content. Video content with sound would require two alternative files; one describing the video part of the presentation, and another describing the audio part.
The descriptions and transcripts can be on the same page, on a separate page. The current convention is to place a hyperlink to the transcript and description near the clip itself.
A number of tools are available that will allow authors to add captions to three multimedia formats: