hello i have a imac 8.1 and my volume down button stopped working..

hello i have a imac 8.1 and the volume down button stop working..

basically it seems that the f11 button is dead cause i can't make shortcut to this button anymore..

i had the maverick osx and i install some vst for ableton before that happend but i make format the disk and clean install the osx lion and it doesn't fix the problem.. any help ??