If you suddenly find that the volume control on your MacBook is greyed out for no apparent reason then you should find that the problem is a relatively minor one, due simply to a glitch in the system. Like all technology, sometimes the simplest things can cause problems in a MacBook, and often the answers are so simple you wonder why you had a problem in the first place. Before you start your investigations, check to make sure that you are able to adjust the input devices you are using. Some will only allow adjustment from the external device.
Whether you are using built in or external speakers, or headphones, the volume of each is controlled by the volume control of your Mac. For external speakers or headphones the simplest thing to do is take out the speaker, or headphone jack, and then put it back in again. Dust can often be the problem, and giving the jack a quick clean, the port a quick blast with compressed air or blowing into the socket yourself, can often be enough to solve the problem. If the jack port is red, your Mac may think there is something connected in there. Try not to be too forceful with the compressed air.
Upgrades can cause problems, and some people suddenly find that they are no longer able to access iTunes or Quicktime. If the volume control is greyed out, it could be worth restarting the coreaudio process. Go into the Activity Monitor, which you can access through Applications and then Utilities. Once you have selected “All Processes” from the menu next to the search bar, simply type in “coreaudio.” Select whichever process is causing the problem, and click on “Quit Process.” Then choose “Quit”, or “Force Quit.” You should find that the coreaudio process will restart itself.
Upgrades may also have an effect on your default settings. If you think this is the cause, open your “System Preferences,” and click “Keyboard.” Uncheck the box, "Use all F1, F2, etc. keys as standard function keys" if it is checked. Once again, give your volume controls a try.
Take a look at Permissions. If these are wrong, they too can cause a problem. Access the “Disk Utility” via the “Utility” folder, which you can access from the “Applications” folder. By clicking “Macintosh HD”, which is on the left sidebar, you will be able to return Permissions to what they should be.