Insert this xml into the <keyboard></keyboard> section of your rc.xml file.

obhud-keybindings-rc.xml

    <keybind key="XF86MonBrightnessDown">
      <action name="Execute">
        <command>obhud --brightness down</command>
      </action>
    </keybind>
    <keybind key="XF86MonBrightnessUp">
      <action name="Execute">
        <command>obhud --brightness up</command>
      </action>
    </keybind>
    <keybind key="XF86AudioRaiseVolume">
      <action name="Execute">
        <command>obhud --volume up</command>
      </action>
    </keybind>
    <keybind key="XF86AudioLowerVolume">
      <action name="Execute">
        <command>obhud --volume down</command>
      </action>
    </keybind>
    <keybind key="XF86AudioMute">
      <action name="Execute">
        <command>obhud --volume toggle</command>
      </action>
    </keybind>