Stereo is my hobby but I still play lp's.
You might look into Gallo speakers. They are round, about the size of a softball, and very high quality. But you might be better of with a "dock" for your MP player that includes powered speakers. It won't be high end hi-fi but I'll bet it will be adequate. I have one in my pool room and the quality of sound surprised me. Boats are not good places for sound so I would not get too elaborate. Not sure what is available that is battery powered though.