The only issue is whether the power supply for the MP3 player can provide enough juice to power the player, charge the player's battery and have enough left over to provide juice to the USB port. USB 2.0 normally can source up to 100 milliamps, although there is a "battery charging" spec that can source 1.5 amps. My guess is your speakers draw the full 100 milliamps when charging, Take a look at the specs on the charger for the MP3 player: if it's only rated at, say, 120 milliamps, it's probably not going to be able to charge both the speakers and player simultaneously and still be able to operate both devices. It's not going to hurt anything, it will just never finish charging.
Safest thing to do is to bring both devices to a full charge before hooking them up. Then the charger for the MP3 player should have no problem keeping them both charged.
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