@gieduduts first make sure that the mic opening is clear of any obstruction like pocket lint etc. Then try to start your phone in safe mode and see if the mic works properly while in that mode. If so, you know that the issue is related to a 3rd party App. Uninstall those and try again.
To place your phone in safe mode do this
# Press and hold the Power button until the Power off prompt appears
# Touch and hold Power off until the Safe mode prompt appears
# Tap Safe mode to confirm
Safe Mode will appear in the bottom left corner of the screen when the phone restarts.
You could also try to get into the hardware diagnostic menu by dialing *#0*# You should now see the diagnostic test menu.
If none of that works, it may be time to replace the mic. The main mic is on the charging board. Since iFixit does not yet have a specific guide for that. Use something like [https://www.youtube.com/watch?v=vVRD7ldy5EM|this video] Replacement charging boards are readily available at many online stores.
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy!