Touch bar volume controls don't work
Solution 1:
Seems to be a bug that shows if you plug an external display, etc.
Open Terminal.app and type:
killall ControlStrip
Solution 2:
The problem is the MAC thinks the external monitors have a speaker when they don't.
Goto System Preferences -> Sound -> Check "Show Volume in the menu bar".
Then from the Volume Bar in the menu bar change the "Output" back to "MacBook Pro Speakers"