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Make sure the volume buttons on the keyboard isn't stuck. If that didn't work, then we can do some troubleshooting.
First reset the System Management Controller of your Mac. Follow the steps below;
Shut down the computer.
Unplug the power cord and also disconnect all other devices.
Wait 20 seconds.
Attach the power cord.
Wait five seconds, then press the power button to turn on the computer.
If that didn't work, then reset the PRAM and NVRAM of your Mac. Follow the steps below;
Shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac boot up.
Please let me know if that didn't work, so we can continue troubleshooting