Iphone 6 Front Cam / Siri not getting voice

Problem - My iphone 6's Front Camera video is not recording sound , Siri not getting commands

but Facetime ,Voicecalls , videocalls ,voice commands & voice memo working fine .

I contacted Apple chat support thrice

First time they told me to Erase all content & setting and set up it as new . Result - Problem remains the same

Second time - open itunes > go to the summary > restore iphone > set up as a new - Problems remains the same

Third time - remove back cover > remove scratch gaurd - problem remains the same

now what to do ? bought it from Singapore n now in India ...