I find there are some cases where my Macbook becomes completely mute, meaning each application cannot make any sound. Even while adjusting the volume, there is no sound during the operation.
One already spotted action that leads to this case is I make it sleep first, and then wake it up. Then, the computer is completely mute. I am sure there are some other cases.
The only solution is restarting the computer.
How may I fix it? Bring it to AppleCare?