A transcript of audio content is a word-for-word textual representation of the audio, including descriptions of non-text sounds like "laughter" or "thunder." Transcripts of audio content are valuable not only for persons with disabilities, but in addition, they permit searching and indexing of that content, which is not possible with just the audio. "Not possible" is, of course, too strong. Search engines could, if they wanted, employ voice recognition to audio files, and index that information; but they don't.
When a transcript of the audio part of an audio-visual (multimedia) presentation is displayed synchronously with the audio-visual presentation, it is called captioning. When speaking of TV captioning, open captions are those in which the text is always present on the screen and closed captions are those viewers can choose to display or not.
Descriptive video or described video intersperses explanations of important video with the normal audio of a multimedia presentation. These descriptions are also called audio descriptions.
The availability of a transcript of audio content satisfies the requirement of the first Section 508 standard formulated by the Access Board.
A text equivalent for every non-text element shall be provided (e.g., via "alt", "longdesc", or in element content).
Sites with audio content that include transcripts meet the Section 508 standards.
For example, many of the segments of the News Hour from National Public Radio include transcripts as illustrated by the following screenshot.
Note that the transcript is not a separate file; it is just part of the page. As mentioned at the beginning of this section, transcripts of audio content are important in allowing your content to be searched and indexed.
The discussion included with the final standards from the Access Board is somewhat misleading.
The Board also interprets this provision [1194.22 (a)] to require that when audio presentations are available on a Web page, because audio is a non-textual element, text in the form of captioning must accompany the audio, to allow people who are deaf or hard of hearing to comprehend the content.
Since captioning of pure audio doesn't really make sense, we interpret the Access Board instructions as requiring transcripts of audio content.