The trigger for the arduino is an analog (I'm assuming) output signal of 5V (100mA) generated by the mp3 player that lasts for the duration of the audio track.
What MP3 player are you using and do you have links to data sheets for it. If your MP3 player is offering a GPI output as it seems from your description then reading on a digital pin should work. Remember you may also need to connect grounds.
If the output is an audio signal then you may be dealing with negative voltages that Arduino pins don't like so be careful.
If the signal is truly analogue (and is positive only) then maybe a RC Circuit
will smooth out the fluctuations so you get a more stable analogue reading.