Volume applet in the tray has gone. How can I get it back?
When you issue a command similar to the following, do you see indicator-sound-service?
ps ax | grep 'indicator'
In case the package was inadvertently removed, try the following:
or
sudo apt-get install indicator-sound
Does this work?
sudo apt-get install indicator-sound
in synaptic search for "sound indicator" and make sure it is installed! then you might have to re login.