The problem has hardware origin. There is a flat cable behind the battery, leading from the upper circuit board, to the bottom small circuit, which contains two microphones, and thle loudspeaker on it. The problem is with the contact on the upper circuit board - it's the wak point. Droping the Phone, or sittimg on it may cause contact fail, and get stuck in flight mode, or deaf microphone, or faulty loudspeaker, ect. You may try pressing the phone's back cover, between the camera and the lower volume button - if it is the contact issue, it may help. Or disassemble the plastic cover above battery, and try to disconnect-reconnect the cabe. It has very tiny precise contacts, so be careful, not to deform them! I had to put some layers of sticky tape on the connector, to have a constant pressure on it when i screwed the plastic cover back. It solved my deaf microphone and flight Mode issue.