HP ProBook 640 G2 Maintenance And Service Manual page 26

Press fn+function key Description
Decreases the screen brightness incrementally as long as you hold down the key.
Increases the screen brightness incrementally as long as you hold down the key.
Decreases speaker volume incrementally while you hold down the key.
Increases speaker volume incrementally while you hold down the key.
Mutes the microphone.
16 Chapter 2 External component identification