I had the same problem and nothing worked for me until I found a handy forum like this wherein someone had mentioned it might be the headphone jack. There's a tiny switch inside the headphone jack that signals to your computer whether to filter sound through the jack or through the speakers. The switch gets jammed occasionally for various reasons. If you've recently had headphones plugged in, they could have jammed the switch, filtering all sound through the headphone jack only (basically, your computer thinks it has headphones in all the time). Test it by plugging in headphones - can you hear sound through them? Then unplug and plug them back in several times to try and unjam the switch. Have a song or something playing so you can see if it resumes working as you do this. Good luck!