In playing a bit more, once I open any site that has audio/video content, in order to access the control to change what's blocked (or not blocked) I have to throw focus to the address box, Shift+Tab once, then right arrow to get to that button.  Hitting enter then gives a dropdown related to that site and its specific permissions, and the autoplay selection is the first in the permissions list.

It appears that all adding/changing of sites is done via this method, while the Autoplay Settings dialog is mostly focused on removing individual sites or all sites for which you've set non-default handling.
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Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041  

It’s hard waking up and realizing it’s not always black and white.

     ~ Kelley Boorn