There have been many requests to control the system level volume, and reserved keys. These are referred to as media keys are system control keys. The original feature request article could be found here.
As of firmware v2.2 and GUI v1.2.0, these features have been implemented and deployed. The following keys are currently supported:
eject, vol_up, vol_down, mute, play/pause, stop, rewind, fast_forward, next_track, prev_track
suspend (this will also wake the machine)
First step, get the latest GUI and make sure you have the latest firmware version. You must be using at least version 1.2.0 of the GUI, and at least version 2.2 of the firmware. To upgrade your firmware, please see the following article.
Now, head on over to the Controllers->Media Keys view.
You should see a screen like the following.
Now pair these keys as you typically would pair any other key. Hover your mouse over any key to get a useful tooltip.