The volume control on your Mac controls the overall system volume and any application you may be using, whether you are using built-in speakers or external speakers or headphones. If the volume control is grayed out for any reason, it could be caused by a temporary glitch, altered keyboard settings, or incorrect permission settings.
Occasionally Mac OS, like any operating system, will experience temporary glitches. This can cause the volume keys to be unresponsive or display a message that you cannot control your volume, even if you were able to previously. These issues are usually not permanent, and often can be resolved by restarting your computer.
Your default keyboard settings may change after an update or system upgrade, which can change the default function of your keys. Open "System Preferences," click "Keyboard," then click the "Keyboard" tab. Make sure that "Use all F1, F2, etc. keys as standard function keys" is not checked. If it is, uncheck this box, then try using your volume controls again.
Permissions control almost everything on your computer, and if any of these are incorrect for any reason, it can affect your ability to use certain functions. Open "Disk Utility" from the "Utilities" folder, which is located inside your "Applications" folder, then click "Macintosh HD" on the left sidebar to allow you to repair all the permissions on your computer to what they should be. This process can take some time, but it may fix any issues preventing you from using your volume controls.
Dusty Output Jack
If anything like dust or dirt is in your MacBook's output jack (where you plug in headphones, for example), your MacBook may believe that something is plugged in whether or not anything is actually inside the jack. If you see a red light coming from inside the jack, this is probably the issue. This can often be remedied by using a can of compressed air to blow out any dust from inside the jack, or by plugging in and unplugging a pair of headphones to try to reset it.
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