1.2.2 Captions (Prerecorded)

(from How to Meet WCAG 2.0 by the W3C)

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.

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How to: Using Caption Services with HTML 5 Video by The A11Y Project explains the HTML 5 captioning options and what the various track files look like. It also recommends a few transcribing services.