The voltage for USB is always 5V. The current supplied by USB differs a bit, the standard allows up to 5A but for unregulated devices (eg chargers) only 1.5A max. An unregulated device basically is everything that has no controller (chip) built in to communicate with the other end. There are devices that ignore this specification though, but these can actually break the host controller on the other end. And in your case the charger would be the host controller anyways (if it had a chip, just because the host always is the supplier).
If your Sony mp3 player really uses this 1.5A (which allready can produce a fair amount of heat in a battery) it would not be bad if the charger only supplies 1A, the batteries would just take somewhat longer to load.
I googled a bit on charging currents for different batteries, and most devices have a maximum of 1.5A. But for small batteries used in such mp3 players as yours 100mA is considered enough, up to 300mA could be used. (example) Of course there are industry standard AA batteries that be charged with up to 700mA, but this is still below your limit and I guess your player does not use one of those batteries. Always keep in mind that in a circular circuit the current through all pieces is the same only the voltage drops after every part, so charging more batteries in line will be no problem, too. As long as you can supply enough voltage (which you can because you supply the needed 5V).