Phone’s sound and vibration stop randomly and come back on

My iPhone 6s runnining ios11.2.6 stops to vibrate and doesn’t play any sound randomly and then these features come back on without doing any thing. The phone charges properly. Whenever I play some media on my phone(when sound isn’t working) the loud speaker makes a crackling sorta sound. Whenever I get any calls the earpiece speaker comes on after around 10 seconds and also when earphones are connected while on call the sound comes on after some time and then it works fine for the duration of the call.Also the media doesn’t play on the phone when there is no sound(ie if I play a video it will keep on buffering but won’t play and it works instantly if I plug in the EarPods). I have done a full restore using restore mode too, it did not help. please help!