In the great tradition of the original Opera classic to go all the way with user interface customization options.
Similarly to a sound mixer where soloing a channel mutes all others.
In Opera, give users an additional option in the Tab settings to automatically unmute current tab, when 'Mute other tabs' is selected.
This way user can use 'Mute other tabs' to silence all tabs when standing on a silent tab, or to hear only that tab - on a currently playing tab.
Final touch: give it a shortcut