Hello, Karansingh Bale.
Thank you for visiting Apple Support Communities.
I understand that the microphone on your iPhone 6s Plus is not working as expected when taking a video. You have tried most of the troubleshooting steps that I could recommend already. However, there are just a couple more that I could suggest in the article below.
Get help with the microphones on your iPhone, iPad, and iPod touch
If the issue persists, see the section quoted below.
If you've tried these steps and people still can't hear you on phone or FaceTime calls, or Siri doesn’t hear your requests, contact Apple Support.
Cheers