In plain language
Audio-only content (podcasts) needs a transcript — video-only content (silent clips) needs a transcript or narration.
Who it affects
Deaf people who need transcripts of audio-only media, and blind people who need alternatives to silent video.
How to check
Find every audio/video without the other channel and check a complete, accurate transcript or audio track is linked right next to it. Worked example — A grocery delivery app's shopping cart: click the transcript link: if it 404s, requires a login the main content doesn't, or otherwise can't be reached, it fails.
Common misconception
YouTube's automatic captions don't count as a transcript, and a transcript link buried on a different page doesn't satisfy this rule. The text needs to be complete, accurate, and next to the media. Real example: on a childcare-booking service's scheduling page, the 'alt text' is not really a description — it's a filename (IMG_0234.jpg) or filler word like 'image', which tells the user nothing.