Sorry, but I'm not an expert on the userchrome.css stuff and this isn't exactly the right place for support requests, especially if they aren't directly related to the addon (i.e. I can't do anything about it by changing the addon's code).
That said, I'm not sure why the workaround I suggested below doesn't work for pinned tabs as it should. People on the Mozilla Support Forum (https://support.mozilla.org/en-US/questions) could probably help you with that though.

Edit:
I found this with a short Google search and it seems to work:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
.tab-icon-overlay[muted], .tab-icon-sound[muted], .tab-icon-overlay[soundplaying], .tab-icon-sound[soundplaying] {
display: none !important;
}

I also added this to the corresponding issue on the Github repository for future reference (https://github.com/abba23/mute-sites-by-default/issues/1#issuecomment-385339727).