여전히 lubuntu 16.04 사용중. 이 환경에서 키보드 hot key를 변경하는 방법이다.
~/.config/openbox/lubuntu-rc.xml 파일을 수정한다. 아래는 내가 수정한 부분 발췌.

화면 잠금 키 변경. 기본은 Ctrl + Alt + l 이었다.

    <!-- Lock the screen on Window + l-->
    <keybind key="W-l">
      <action name="Execute">
        <command>lxsession-default lock</command>
      </action>
    </keybind>

audio 제어

    <keybind key="W-F8">
      <action name="Execute">
        <command>pactl set-sink-mute @DEFAULT_SINK@ 0</command>
      </action>
      <action name="Execute">
        <command>pactl set-sink-volume @DEFAULT_SINK@ +3%</command>
      </action>
    </keybind>
    <keybind key="W-F7">
      <action name="Execute">
        <command>pactl set-sink-mute @DEFAULT_SINK@ 0</command>
      </action>
      <action name="Execute">
        <command>pactl set-sink-volume @DEFAULT_SINK@ -3%</command>
      </action>
    </keybind>
    <keybind key="W-F6">
      <action name="Execute">
        <command>pactl set-sink-mute @DEFAULT_SINK@ toggle</command>
      </action>
    </keybind>
    <keybind key="W-F5">
      <action name="Execute">
        <command>audacious -t</command>
      </action>
    </keybind>

참조 : https://help.ubuntu.com/community/Lubuntu/Keyboard