There are multiple microphones on that device. The one that Siri uses is the upper mic. When you talk on the phone normally, it uses the mic that is next to the lightning port. That one is probably toast. A replacement charging port flex will probably fix it. It replaces the microphone down there.
You need to either open it up and clean the corrosion out of it or take it to a repair shop. The battery needs to be replaced no matter what since water damage usually toasts the battery right away. As for the flash, the corrosion might be shorting it making it stay on all the time.