The KDE version:
System Settings -> Shortcuts and Gestures -> Custom Shortcuts. Click on Edit -> New -> Global Shortcut -> Command/URL and enter the shortcut under the Trigger tab. You'll (most likely) see a dialog warning that the key already is assigned to a global shortcut, choose Reassign. Finally write the command you want to run under Action.
dbus-send --dest=org.gmusicbrowser /org/gmusicbrowser org.gmusicbrowser.RunCommand string:PlayPause
Repeat for the other keys.
System Settings -> Shortcuts and Gestures -> Custom Shortcuts. Click on Edit -> New -> Global Shortcut -> Command/URL and enter the shortcut under the Trigger tab. You'll (most likely) see a dialog warning that the key already is assigned to a global shortcut, choose Reassign. Finally write the command you want to run under Action.
dbus-send --dest=org.gmusicbrowser /org/gmusicbrowser org.gmusicbrowser.RunCommand string:PlayPause
Repeat for the other keys.