when i use kmix, i can mute/unmute and change the volume with my laptop keys normally.

However this doesn't work when i only use the plasma-pa applet while having kmix disabled, so i know it is not that the button(s) are incorrectly recognized, since kmix recognizes them.
This did happen on plasma 5.6 as well (didn't use plasma before 5.6)

Let me know if i should provide logs or similar. 

Regards

Reproducible: Always

Steps to Reproduce:
1.Disable kmix (if not already disabled)
2. Try to change volume or mute sound with dedicated keys

Actual Results:  
nothing happens

Expected Results:  
i should be able to change sound settings through machine hotkeys

Samsung NP550P5C-S02 with hardware volume hotkeys:

MuteSound=Fn+F6
VolumeUp=Fn+F7
VolumeDown=Fn+F8