Some times it happens while making some update to some other User interface like Cinnamon.
Sound applet or icon from the right top panel of ubuntu 11.04/12.04/12.10 doesn’t appear but sound still keep on working.
This way we can add sound applet again to the panel:
1. Open Terminal and write these commands one by one:
2. Install missing Sound indicator applet :
sudo apt-get install indicator-sound
3. Refresh Unity using this command: