Hi, I thought the purpose of that Site information BLOCK/ALLOW button was to reflect the users choice of Blocking or Allowing a website to autoplay media with sound, that being said I think it should have priority over any other settings the user had in the about:preferences > Permissions section, regardless of the Exceptions he manually added to the list.
Since the functionality for the about:preferences > Permissions > Block Media with sound > Exceptions is that when you add the same website with a different status it just changes the status instead of removing the exception entirely, it should behave the same from the Site information panel specially since it reflects the user's choice.
It does not matter if the user disabled the Global Preference altogheter if he manually Blocked or Allowed media with sound it should take priority over any other settings, and the exceptions list should not change until the user feels otherwise.
Plus there is also the case where the user disables the Block autoplay feature but would still like to block some websites when he reaches them, option that is no longer possible from the site info panel , only if he reaches about:preferences > Permissions > Exceptions list.
In this case when the user Allows or Blocks a website from playing from the Site information panel and the Website is removed from the exceptions list, if he disables the feature at a later time he has no way of knowing why the websites he manually blocked are no longer blocked from playing media with sound, that is why the exceptions list should take priority over any other settings when a user's choice is involved and it should reflect those choices from the Site information panel.