As I'm getting Lubuntu back to how it was before, I noticed that my volume keyboard shortcuts don't work. The Fn keys definitely do their job, but for toggling mute and adjusting the volume level, nothing happens.

I try to remember what I did to fix it, and with some google-fu managed to find the Ask Ubuntu page that helped me before.

First we need to check what is currently configured in Openbox's configuration file.

$ sudo vim ~/.config/openbox/lubuntu-rc.xml

Inside is a long list, so let's jump to "Volume management" using Ctrl+F:

...

<!-- Keybinding for Volume management -->
<keybind key="XF86AudioRaiseVolume">
  <action name="Execute">
    <command>amixer -q sset Master 3%+ unmute</command>
  </action>
</keybind>
<keybind key="XF86AudioLowerVolume">
  <action name="Execute">
    <command>amixer -q sset Master 3%- unmute</command>
  </action>
</keybind>
<keybind key="XF86AudioMute">
  <action name="Execute">
    <command>amixer -q sset Master toggle</command>
  </action>
</keybind>

...

We can see if that keybinding is indeed correct for the volume control keys by using xev:

$ xev

Then while xev is running, press the keyboard shortcut (I tried Fn+F1 for the mute toggle). The keybinding does indeed show up as XF86AudioMute.

So referring back to that Ask Ubuntu solution, it seems the command needs to be adjusted. The original:

<command>amixer -q sset Master toggle</command>

Now the new command should read:

<command>amixer -D pulse sset Master toggle</command>

Put it together for all the bindings we need and we have:

...

<!-- Keybinding for Volume management -->
<keybind key="XF86AudioRaiseVolume">
  <action name="Execute">
    <command>amixer -D pulse sset Master 3%+ unmute</command>
  </action>
</keybind>
<keybind key="XF86AudioLowerVolume">
  <action name="Execute">
    <command>amixer -D pulse sset Master 3%- unmute</command>
  </action>
</keybind>
<keybind key="XF86AudioMute">
  <action name="Execute">
    <command>amixer -D pulse sset Master toggle</command>
  </action>
</keybind>

...

Save the file, close it, and remember to recongifure Openbox:

$ openbox --reconfigure

All done!