Creating a new sufficient technique using audio ducking to create audio descriptions

[mbgower]

The fairly long discussion in #1768 surfaces the idea of using audio ducking to provide more opportunities for audio descriptions within a prerecorded video.

Introduction of the same technique is also proposed through PR #1790 to the existing language of 1.2.5

Although this may work better as a discussion, I thought I would initiate it as an issue.

[mbgower]

@mraccess77 made the following comment in 1768:

I worry about audio ducking reducing the volume of audio and playing two audio tracks at once for those who are blind/low vision who are also hard of hearing. I would think that any non-overwritable ducking should be limited to non-spoken audio or important sounds.

[patrickhlauke]

regarding @mraccess77 's comment: absolutely, i thought it was implicit that you'd only duck parts that don't have spoken words/content. worth making it explicit though, just in case