I have Ubuntu 12.04 and I just logged in and noticed that the sound icon on the top Unity Panel is missing! How do I get it back?
If you are on a laptop, the sound icon will disappear when you reboot with the volume on mute and it will usually return if you use the fn buttons to adjust the volume.
Another way to remedy this type of situation is to bring up your system settings to change the volume from there.
Next, check to see you have a file under "/etc/pulse/default.pa". if you do not have this file, you need to download a copy of default.pa see the answer to this post: How to make pulseaudio work again? Reboot after you replace the file.
3 (Accepted Answer)
Finally, check to see if or reinstall "indicator-sound" and "pavucontrol" and maybe the indicator applet packages as well.
Also, if I remember correctly, there should be a panel volume control in "software center" but I've never had that prob in 12.04 yet