Hey,
This is a fritzing diagram and a schematic of my circuit:
Note: I have put a speaker instead of the hacked MP3 player in the circuit. What i've done is a crude override of the on/off switch using a relay to turn it on/off remotely. I have only included the board with the relay on it.
The mp3 player is from amazon: http://www.amazon.co.uk/gp/product/B004UYTA08/ref=wms_ohs_product and powering it on and off with a relay. It has its own power supply. I would post the mp3 power details but they're unknown to me. I've tried a few different ways of doing this remote control hack, but it is now proving temperamental. Today i've gone to re-test the circuit and its not working properly. I do think it is interfering with the power source as you mentioned before. It wont stop once its started playing now for some reason.
Thanks,
Siobhan