Turn off the mic while you are in the lobby and switch it on when you are in the plane if it doesn’t solve your problem then follow these steps
1) UPDATE THE GAME
There are often smaller updates which are mostly bug fixes which go unnoticed, but it is a likely cause of the mic fail. hence can be prevented by updating the Game
2) Give permission to use Microphone in the APP settings
Go to settings-> APPS-> select PUBG MOBILE->Permission->Turn on mic
3) Reconnect to WI-FI
Make sure the router is running on latest firmware from the manufacturer
Try turning off SIP ALG in the router setting. this will help in fixing the bug or you can disconnect the wi-fi and then again connect to it but this will be a temporary fix if it doesn’t work close the game, disconnect wifi and then connect it again. it will work.
4) Emulator mic problem
if you are using headphone and your laptop/PC have have mic installed then right click on the sound icon in right hand corner and select playback devices then go to recording tab and turn off the laptop/PC mic so that the game uses your headphone mic only.