You can use the volume keys on your keyboard (generally same as the F11 / F12 function keys) to control the volume of your monitors with built-in speakers. To enable this option:
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Click on the Settings Icon in DisplayBuddy > Preferences > Keyboard Shortcuts and check “Use Volume keys to control volume for all displays”.
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macOS requires you to give DisplayBuddy additional permissions to utilise your volume keys. You’ll need to provide Accessibility permissions to DisplayBuddy via macOS System Preferences > Privacy & Security > Accessibility > Enable DisplayBuddy.
The video above also shows you all steps needed to enable volume control using volume keys on your keyboard.