To fix Mute button keyboard shortcut issue on LXDE

Create a script call "amixertoggle" and save it in /usr/local/bin:

#!/bin/bash

amixer $1 sset Headphone toggle
amixer $1 sset Speaker toggle
amixer $1 sset PCM toggle
amixer $1 sset Master toggle



Edit file $HOME/.config/openbox/lxde-rc.xml and replace block that has "XF86AudioMute" to call our script.  For example:

...
...

    <keybind key="XF86AudioMute">
      <action name="Execute">
              <command>amixertoggle -c 1 toggle</command>
      </action>

...
...

(" -c 1" above is for machine, where the mixer control is actually on card 1.  If doesn't work, we can try different number)

Reload the modified file lxde-rc.xml by doing:

openbox --reconfigure

That's it.  Everytime we press "Mute" button on our PC keyboard, the mute/unmute will toggle.