Some time ago I made an emergency mute button that connected over USB. Now with Adafruit's Bluefruit EZ-Key I made a wireless version that works over Bluetooth. Look, no wires:
In addition to the Bluefruit board, I used a Trinket and a 100 mAh lipo battery. It all fit nicely inside the button:
Here's a diagram of the connections:
There's no on/off switch and to charge the battery I have to disassemble the whole thing, but hey, nobody's perfect. Here's the Arduino sketch that's running on the Trinket, it simply sends the "mute" key code when it detects a change in the state of the button. I'm using this SendOnlySoftwareSerial library, because I'm only sending data to the Bluefruit board.
(You will notice that it still suffers from the synchronization problem that the USB version had - if you mute audio on your computer some other way than using the button, then pressing the button will actually unmute audio. It's because the button is stateful, but has no way of finding out what the actual state of audio on the computer is and there are no separate key codes for muting and unmuting.)