Hi I also have this problem. Sound was only working on the headphones and also on the volume slider in the settings app. Everywhere else I get no sound and no volume slider. Now the headphones don't work either.
(I'm using an ipad 2 iOS 6.1.2)
And NO it's not muted. I've reset all settings and restarted the iPad. Been searching forums for a couple days. Some say its a dirty docking port problem, others suggest a Skype or other app update that could be the problem.
No volume control icon appears when I press the volume control buttons. Only says sound effects, if it says anything at all. Most often there is NO volume icon. YouTube, video, music, and other game apps, no sound.