@michinyon,
Actually, the signal the OP wants to use is of UNKNOWN nature and moreover, it is not at home because it is on a job site. The audio signal which fits the signal parameters you mentioned is not going to the arduino, but the mp3 player supposedly has a separate (it seems) connector that outputs a signal while the music is playing and changes state when the music stops. We have not been given a name or any info on this signal or even any info on the mp3 player so we're basically just trying to cover all the bases. This is all we have:
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.
Riva suggested it is GPI (no idea what that is) There is no suggestion at this point that it is negative or has negative transistions, but the positive signal type is also unknown except for the level is 0 to 5V , which sounds like TTL digital (an not analog) to me.