The change you used is only a partial solution to a different problem. It is when there is more that one audio device (such as your graphics card having HDMI audio out), which might cause feedback, squealing etc. Try booting back into your system using the boot stick - plug it in with your unit off, start it changing to one time boot sequence (f12), hit any key when prompted, select ML (not installer), use boot flag -v (or -v -x if you were getting KP) and see if you can boot into the system. See if the volume control you are looking for has been placed in a new bottom row of System Preferences panel). If so, undo changes, run utility & reboot. If you can't get in with the boot stick try booting in with -s.