There are two reasons why the sound on a smartphone does not reach the person you are talking to. The first reason is that the inside of the microphone is clogged with dirt, dust, cotton wool and similar particles. The problem can be solved by cleaning the microphone socket. The second reason is the failure of the microphone socket. Impacts can cause the microphone socket to break or crack. Or it can oxidise due to contact with liquid or moisture. In such a case, the microphone socket should be replaced.