Volume Disappears.

About 50% of time using my iphone, it does not allow me to control the volume. It sets itself to mute and hides all volume control options.

There is no ringer volume, no volume control in music player nor videos, notification centre stops also.
The phone is not set to mute. U may be hearing a song and suddenly the volume bar disappear and the volume as well.This happens since the last software update.