A 'typical' solar panel will be around 10% efficient. Full sunlight is about 1kW per square meter. So a panel 1m X 1m will give about 100W.
Now find out how much power the mp3 player requires to run and calculate how much power that panel will give. Bear in mind that I doubt it has a switching regulator and it claims to drive at 9v, so it's nominal voltage is probably around 12v so the max current will be W/12 irrespective of it's output voltage.
Aside from this, inbuilt solar chargers are always prone to climatic, environmental & solar wrath harming the device itself. Avoid them ! I'd a solar charging radio once that (even behind a window indoors) became so hot that the electronic innards became totally cooked! Of course a sudden downpour outdoors would give ruinous damage too, & light fingers/animals may even steal it. Best overall IMHO to keep the PV well seperated from the device.