Sorry for digging out an old post, but I have found a solution
In my case it was an issue with actions assigned to
XF86AudioLowerVolume
XF86AudioRaiseVolume
in the file
~/.config/openbox/lubuntu-rc.xml
Try to find a command that will work for lowering/raising volume from the terminal, in my case it is
amixer -D pulse sset Master 5%-
amixer -D pulse sset Master 5%+
so my entries in the file look like this (I left the default lubuntu entries commented out)
<keybind key="XF86AudioRaiseVolume">
<action name="Execute">
<!-- <command>amixer -q sset Master 3%+ unmute</command> -->
<command>amixer -D pulse sset Master 5%+</command>
</action>
</keybind>
<keybind key="XF86AudioLowerVolume">
<action name="Execute">
<!-- <command>amixer -q sset Master 3%- unmute</command> -->
<command>amixer -D pulse sset Master 5%-</command>
</action>
</keybind>
This whole section is inside the element (as per https://wiki.lxde.org/en/LXDE:Questions#How_do_I_make_my_special_keyboard_buttons_.28mute.2C_volume.2C_screen_brightness_....29_work.3F )
You may need to logout/login or restart after making changes to the file.
Hope this helps.