Guideline 1.2: Time-based Media - Provide alternatives for time-based media.
Success Criterion 1.2.2: Captions (Prerecorded) - Captions are provided for all prerecorded audio content in synchronized media, except when the media is a media alternative for text and is clearly labeled as such.
Success Criterion 1.2.3: Audio Description or Media Alternative (Prerecorded) - An alternative for time-based media or audio description of the prerecorded video content is provided for synchronized media, except when the media is a media alternative for text and is clearly labeled as such.
Success Criterion 1.2.4: Captions (Live) - Captions are provided for all live audio content in synchronized media.
To begin, it's important to understand what time-based media is. The Guggenheim Museum aptly describes the media as one that "unfold[s] to the viewer over time." [1] For our purposes, and according to the WCAG, this includes media that is audio-only, video-only, audio-video, and audio and/or video combined with interaction. On your guides, this would include videos or video tutorials (such as those from YouTube), interactive modules that include any audio or video (including something as simple as a chime to let users know when to proceed), and any audio- or video-only files (such as a recording of a speech or a video-only recording of a PowerPoint presentation).
Captions and transcripts are useful to people who are deaf or hard of hearing, but they can also be useful to those that have difficulty processing auditory information, are not fluent in the language in which the the media is presented, and those that cannot access the auditory information (e.g., they're working in a quiet space without headphones).
Descriptive transcripts (those that provide textual details about the audio and video) are useful to people that are both deaf and blind since the information can be transcribed in braille.