This is a idea/plan that I want to do.
Idea comes from the computer I'm using. My old computer (HP TX1106au) has a physical mute button and you just push it and it worked. But my new computer (ASUS U36sd), will, if I want mute, I need to put Fn key and F9 which means mute, but it only works when all the system is boot up because it depends on the software to function which I don’t like.
As I know, as a HID input device, mute has a designated key value Dec 127/Hex 7F (you can find it at: http://www.usb.org/developers/devclass_docs/Hut1_11.pdf on page 56). And this blog (http://hunt.net.nz/users/darran/) give me the idea to build a usb input device though I may not use arduino in the final model. What I may use is V-USB (http://www.obdev.at/products/vusb/index.html), and the chip maybe ATMEL attiny85, the ideal size is about a Logitech Unifying Receiver.