@Chrysostomus I decided to include them directly in the lxde-rc.xml file with sh -c"..." commands. You can see the latest commit in this file: https://github.com/ThanosApostolou/desktop-settings/blob/lxde/community/lxde/skel/.config/openbox/lxde-rc.xml (I haven't done any pull request yet). Apart from the lxde branch on my forked desktop-settings repository, I also have the previous lxde-rc branch in which I have created separate scripts. Both ways have the same outcome.
However, on my 1st laptop it works fine, but on my 2nd it doesn't work always. I keep logging out and logging in and 1/10 times it works fine (audio changes by 5% and shows notification) but 9/10 times it doesn't work (audio changes by 2% and no notification shown). I've spent many hours in trying to figure this out, but I cannot explain it. I'm testing on live session because I don't have any empty disk space in order to do an installation but I don't know if it changes anything.
In order to see it easier, these are the changed openbox/lxde-rc.xml lines:
<command>sh -c "pulseaudio-ctl up; notify-send \"Volume Increased: $(pulseaudio-ctl current)\""</command>
<command>sh -c "pulseaudio-ctl down; notify-send \"Volume Decreased: $(pulseaudio-ctl current)\""</command>
@Linj can you try to download the new file from here https://raw.githubusercontent.com/ThanosApostolou/desktop-settings/lxde/community/lxde/skel/.config/openbox/lxde-rc.xml and move it to ~/.config/openbox/lxde-rc.xml, then logout and login (or even better reboot) and tell me if it works for you?