Audio files always need transcripts or text equivalents.
Video files always need transcripts and/or captions of dialog.
Captions should be:
WebAIM has more information about Captions, Transcripts, and Audio Descriptions
There are differences in how these text descriptions are delivered:
Read these tips on how to create captions.
These tools are mainly for video captions, but can be used for audio files as well:
Additional options are available on Captioning Your Own Videos (For Free)
Some of these sites will output the text as transcriptions.
See also: Formatting Transcriptions
Audio Description (AD) is way to describe visual content to a visually-impaired audience.
In web videos, they are often embedded as separate audio tracks that can be toggled ON/OFF.
The Audio Description Project from the American Council of the Blind compiles lists of museums, performing arts venues, DVDs, TV channels, streaming services, and more that provide audio description.
Although AD is not necessarily required by law, please use it in your professional practice. Learn more with the Describing Visual Resources Toolkit
Daredevil is a television show about a blind lawyer who uses enhanced senses to navigate the world. Yet Netflix did not consider that any of its viewers would be visually impaired.
Find out more about the fight for audio description for Daredevil.
Thomas Reid describes what happens when audio description is done badly in this podcast episode (text and audio). The AD track for Black Panther: