battery-in or a washed phone is bad.. the electrical current, combined with the water is what causes the corrosion.. disconenct the battery, clean the board with soapy water and a toothbrush, followed by clean water, followed by alcohol (not vodka! neat alcohol! go to a chemist!), then leave to dry for a few days in a warm place, or use a hairdryer

the mic is broken totally? understandable. It should be replacable with any small microsphone though, check your local radio spares/maplin/radioshack etc