I'm using the custom action above, but it doesn't work how I want. It really does seem to pause the audio, rather than mute/disable the audio listener -- so all audio clips that have been triggered after the Mute button was clicked are queued up, and they all play when you unmute the audio.
I think I'd be fine with simply disabling/enabling the audio listener, but I can't figure out how to drag that component specifically to the FSM state -- when I click on the Audio Listener GO, the FSM disappears from the Playmaker panel, so there's nowhere to drag it to...