Hi
Yes you are right it is very frustrating, first the the tab is so small that you need to be an experienced surgeon to click the sound button and the minuscule corner to open the tab, and secondly and most importantly the mute and unmute capability is lost when moving between the pinned tab and other tabs.
This issue was recently discussed here Mute / Unmute Pinned Tab is Difficult/Annoying - #11 by Chocoholic and GitHub was contacted about the issue and hopefully we will see a solution to this in the near future.