How can I mute audio using external non-Apple keyboard?
The only thing I could think of is making two services (mute + unmute) through Automator and then assigning these services to two different shortcuts from the settings panel.


First open Automator and choose service, then add a shell from the actions menu 
Next write the shell script to mute/unmute and save them 


To mute : osascript -e "set volume with output muted"

To unmute : osascript -e "set volume without output muted"




Finally go to Settings -> Keyboard -> Shortcuts -> Services, find the two services you created and assign the shortcut you want.