Drag the shortcuts to your shortcuts bar to have a way to mute or unmute <audio>. I may swap to a toggle later....

Mute Play

For you nerds, the code looks like this
javascript:(function() { var sounds = document.getElementsByTagName('audio');for (variable in sounds) {sounds[variable].volume = 0;}})();

paal.org / twitter.com/PaalSA / 2011