Nokia Lumia 820 Mic Solution Jumper Ways
https://gsmfixer.blogspot.com/2015/09/nokia-lumia-820-mic-solution-jumper-ways.html
Mic problem in Nokia Lumia 820 can be occur due to the missing points of mic or water damage on the board sometime mic itself stop working so you can solve the problem by replacing the mic with new working on and clean the board with thinner or CTC.
If not solved the you have to repair the problem by making jumpers. Here is the hardware repair diagram solution for Nokia Lumia 820 mic problem.