I can't work out from the description whether this 3 volt module is being switched on the high side or low side.
Collector connected to MP3 module ground.
Can you supply a diagram ?
Ideally you'd switch it on the high side and use a pnp transistor (or p channel logic level mosfet ) and connect it like this if your Arduino is a 5 volt device:
This circuit is suitable only if the voltage you are switching is lower than than that of the Arduino. It is also inverted so a LOW on the arduino pin switches the module ON.