3 Answers

Are you certain you don't have it muted, or the volume turned completely down? Have you tried plugging ear phones into the audio jack? 

The system preferences /sound files should be checked first. Then progressively through the programs you use to check the sound level about mid range on all control settings. The problem is that you are going from one program to another and on one or more the sound is down. Funny how system files go to a subsystem file and because the files are daisy-chaining permissions and settings you get to a point where nothing works. Apparently programmers have a tough time with this, seeing as it is so prevalent. 

You could rebooth your system or if you have speaker make sure there turn on.