What happened to me was i inserted my headphones in the plug while the net was still loading. Apparently my sreen froze on me. I shut down the power and restarted it. After this process, i lost the sound. I couldn't even hear the apple sound when i restart the computer. if i go to system prefference i can open up the sound, but it won't allow me to adjust the volume. everything else in the sound prefference is transparent. I couldn't play itunes.