If the packages
xfce4-volumed are installed you shouldn't have any problem:
This is the frontend for mixer settings delivered together
with the Xfce4 desktop environment. It does the same jobs
other mixer frontends do but is integrated into the Xfce4
desktop as a plugin for the Xfce4 main panel.
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.
Edit: I was also searching for a definitive answer and ran across this http://forum.xfce.org/viewtopic.php?pid=26844 which also lead me to see this https://github.com/smatting/pulsemixer
Now, the problem "Volumed points to sound card, and when I mute with Fn+keyboard, it mutes pulseaudio but cannot unmute it"!
So this issue was Solved using this application PULSEMIXER