Install xfce4-volumed by entering the following commands in the terminal:
sudo apt update sudo apt install xfce4-volumed
Description:
volume keys daemon
This is a volume keys control daemon for Xfce Desktop environment. It controls the volume using multimedia keys, for the card configured in xfce4-mixer. It also provides volume change notifications.
Homepage: http://goodies.xfce.org/projects/applications/xfce4-volumed
Version: 0.2.0-0ubuntu2
Section: universe/xfce