Top line of buttons on keyboard have stopped controlling the volume

Go to System Preferences->Keyboard

and make sure that “Use F1, F2… as standard function keys” is NOT checked.

As perviously suggested.

Question - this is computer using and External Display and over HDMI cable ?

Using a Mac Mini M1 with Asus 27" Display over HDMI and all controls of Brightness and Sound are passed to the Display Controls.

Therefore the fn keys may function - but do not actually have a Real Control