F7, F8, F9 media keys no longer works in High Sierra 10.13.1
First I would prefer to restart Mac system.
you can change your function shortcut keys.

Change the behavior of function keys on your Mac

You can use the top row of keys on your Apple keyboard as standard function keys or to control built-in Mac features.

If your Apple keyboard has icons printed on some of the keys on its top row, 

these keys can be used to perform the special features shown in each icon. They can also be used as standard function keys (F1–F12). 
The action performed by standard function keys varies depending on the app you're using, or depending on the keyboard shortcuts listed in the Keyboard &amp; Mouse pane of System Preferences:
By default, pressing one of these keys performs the special feature indicated by the icon printed on the key. For example, pressing the keys with speaker icons adjusts the volume.

If you prefer the top row of keys to always behave as standard function keys without holding the Fn key:


Choose System Preferences from the Apple menu.
Click Keyboard. 
Click the Keyboard tab if it's not already highlighted. 
Select "Use all
F1, F2, etc. keys as standard function keys"


With this option enabled, the keys on the top row act as standard function keys (F1–F12). To use a feature indicated by the icon printed on a key, hold Fn while pressing the key.