...plus you're not able to change the volume level.
First of all, make sure "alsamixer" is installed and working properly
Open the terminal (Ctrl + Alt + T) and type
alsamixerIf you can see the mixer and change the volume from there...awesome! You'll only have to reinstall the sound indicator.
To do this...exit alsamixer (press ESC) and type on the terminal
sudo apt-get -f install --reinstall indicator-sound indicator-sound-gtk2After reboot you should see the widg
et working again.
In case your "alsamixer" is not working properly but it's installed...try to force the reload by typing
sudo alsa force-reload
If instead, it's not installed anymore...just re-install it
sudo apt-get install alsa