Brightness and volume buttons don't work

Solution 1:

Go to the KDE settings ("configure desktop" or "system settings") -> Shortcuts and Gestures -> Global Keyboard Shorcuts.

For volume, go to KMix, click "Volume Down" next to "Decrease Volume", click the box next to "Custom", then hit "Volume Down" button on your chromebook. Do the same with "Mute" and "Volume Up".

For brightness, go to "KDE Daemon". Set "Decrease Screen Brightness" and "Increase Screen Brightness" in the same way.

If the buttons are not detected at all, then it is likely that X11 does not support the buttons.

Solution 2:

Okay so brightness works now. With audio it took me a while to figure it out but basically you need to install pulseaudio FIRST, reboot, and then install KMix and reboot again. Now it works :D