Anything is possible with the right thinking and the right amount of money
The buttons are for the crappy internal speaker, but since they are buttons, they could be used for other purposes. Depends on your knowledge of electronics and wiring.
I personally could not tell you how to use them for changing the system volume, but I'm sure there are plenty of people here that could. My thoughts are either a) use them to connect the volume pins on a SB card (search for the thread in Input Devices regarding the SB cards that have this header) or b) make some sort of IR sending unit that responds to these button presses and send IR signals to an IR receiver on the PC, which then you can use Girder to control the system volume.