Why my Nokia Lumia 920 phone's sound system is dead suddenly? What is the solutionto recover sound

[Windows Central Question]

My above phone has gone suddenly dumb and deaf. No music, no call ringtones, no skype, no Cortana - nothing related to voice and sound. My phone has not been dropped, sunk in water or anywhere near magnate or hot appliances, it is always in soft cover. I have taken utmost care yet why this happens