The audio listener is simply a component, so use the action "set property" drag the audiolistener component ( not the gameObject, really drag the audio listenre component itself), then select the "enable" property and turn that on and off.
I just tried this and, oddly, it worked in the editor but not in my compiled game. I tried both "set property" and "enable behavior" actions. Both gave the same result. In order to mute all audio, I had to attach this script to my camera with the audioListener, and then invoke one of the two methods using the "invoke method" action.
#pragma strict
function muteAudio() {
AudioListener.pause = true;
}
function unmuteAudio() {
AudioListener.pause = false;
}