As I started to use the i3 window manager I wanted to have a visual feedback on sound volume change events. Usually it's done by a daemon that binds to specific keyboard combinations to adjust volume and to send notifications to a notify daemon. In xfce this is done by the xfce4-volumed. I should also point here to other lightweight volume daemons like volnoti or pa-applet. Without a dedicated daemon it is also pretty much simple to handle sound volume keys by a simple bash script which is bind to specific keyboard combinations.