For https://www.w3.org/WAI/WCAG22/Understanding/audio-only-and-video-only-prerecorded.html if a page uses a decorative video (like an animated background) or audio (some muzak, drone sound, etc) only to set the mood, it would likely be exempt from needing an media alternative - as there is no "information" conveyed by it that would require "equivalent information" to be delivered.

Thoughts? Happy to do a PR if we have agreement...