Hi @himanshu2, I am sorry to hear about what has happened. 
Have you had the chance to test the microphone while recording a video and then watch the video to see if there's sound, or in the Diagnostics test? You may find the test in Diagnostics under Settings > About phone > Diagnostics > Test > Microphone, can you record something and play it back here?
If you have the chance to, please also try to see if it persists while in safe mode, this will let you know if there could be a third party app causing the behaviour.
Let me know how it went! 
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