If you look at the bottom of your Nexus 5x you will see a pin hole that allows the microphone to receive your voice. The microphone component will be right above that small hole. Unfortunately, I believe that the microphone component itself is soldered to the motherboard. You may be able to clean it more with alcohol and try again after it dries. Buying a new microphone and soldering on the new one would require a special soldering iron and almost expert skill. If you could purchase one with a broken screen for a reasonable price you could replace your whole motherboard with ease