Turn off the LG G4, remove the SIM card and then reinsert the SIM card on turn on the smartphone.
Dirt, debris and dust could be stuck in the microphone, try cleaning the microphone with compressed air and check to see if the LG G4 audio problem is fixed.
Audio problem can be caused by Bluetooth. Turn off the Bluetooth device and see if this will solve the audio problem on the LG G4
Wiping cache of your smartphone can also solve the audio problem
For more fixes check LG G4 Manual- guideusermanual.com/product-name-lg-g4-manual&po=325291&lang=English