Without knowing anything about that mp3 player, i'd say carefully disassemble it so you can inspect and clean all the parts. Knowing what's happening inside might make fixing it possible. Hopefully it's the kind of thing that you can disassemble, held together by screws rather than glue. Keep track of where all the parts go, take photos of every step if that would help.
Best case is probably that you just need to let it dry out and then you can put it back together. Worst case is something essential melted. Probably the truth is somewhere between those two.