According to success criterion 1.2.6 (AAA), sign language interpretation should be provided for all pre-recorded audio in synchronized media.
People who are deaf or hard of hearing may use sign language as their first language and have limited reading ability which makes it harder to read and comprehend captions in synchronized media. For them, therefore, sign language is faster to interpret and is more descriptive in terms of intonation and emotion compared to captions. However, note that sign language differs in countries and not everyone with hearing impairment understands sign language.
To comply with the criterion, you could add sign language interpretation to the video that is presented to all users, or provide a link to a video that has sign language interpretation.
Success criterion 1.2.7 (AAA) talks about Extended audio descriptions and is an extension of 1.2.5 (AA) Standard audio description.
According to WCAG 1.2.5 (Level AA), pre-recorded videos that have sound should have an audio description. However, sometimes the content doesn't have long enough natural pauses for the audio narration to be added. In this case, to comply with AAA, you may need to pause the video to allow the extended audio description to be added. The video then can resume once the description ends.
Success criterion 1.2.8 (AAA) states that an alternative in text form (most commonly as a transcript) should be provided for any audio-visual content. This is important because people who may not be able to read captions and hear sound have access to the same information. The text transcript should include full descriptions of the audio and visuals including visual context, actions and expressions of actors, and any other key visual material. In addition, all spoken audio like laughter and off-screen dialogues, on-screen text should be included in the transcript.
Success criterion 1.2.9 (AAA) states that alternative text should be provided for information covered by live audio such as meetings, conferences, podcasts etc. This can be achieved through a real-time captioning service or a transcript if it’s a prepared script, for example, a pre-written script for a live press release.