Make audio and video related content fully accessible to vision and hearing impaired users.
Video Content with Audio
Users who are deaf, hard of hearing, or having trouble understanding the audio information must be provided with a text based alternative.
Captioning should be provided for the audio portion of a video.
The captioning must be synchronized with the audio so that someone reading the captions could also watch the speaker and associate relevant body language with the speech.
Example:
https://www.w3.org/WAI/media/av/captions/
Audio-Only Content with No Video
Audio files with no video are not considered multimedia. However, since audio is a non-text element one of the following text-equivalents must be provided for deaf or hearing impaired users.
HTML based transcript
Text transcript
A text or HTML summary
https://www.w3.org/WAI/media/av/captions/