If you had trouble locating the actual extension, like I did, here’s a direct link: http://www.amugticino.net/download/Audio_Switch.safariextz
This toolbar extension add a button that allow the user to switch off/on the audio of the browser just clicking on it. The user can choose if the audio will be enabled or disabled by default.
Made by effeerre75 for the AMUG Ticino groups.
Note: website in Italian.