Comment 4 for bug 1404468

Actually this seems to affect every key that is not handled directly to the target application, but indirectly (through the window-manager?), e.g. also media buttons when the associated media application window is not focused.

I also seem to have stumbled over a workaround somehow: I'm using Ubuntu Gnome here, and after installing kubuntu-desktop (during installation I selected lightdm, not gdm) and starting into KDE once (there the affected buttons didn't work either), on next login the login screen changed to KDE-style (where you can choose the UI type). After logging in to Gnome again the buttons seem to work.