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.
Let's try to help the OP solve his problem with a minimum of fuss.
It's not clear from this quote whether the MP3 player has a special signal output to indicate when a track is playing or whether the OP is hoping to detect the existence of music as a means to trigger his relays.
If there is a special output signal (which is something I haven't heard of) it is likely to be a simple 0v or 5v digital signal so it should be easy to detect on an Arduino digital pin.
If the OP is trying to detect the music signal I think he will need to rectify it with a diode and then use the analogRead() to find out what sort of stuff is detectable. It will be difficult to get useful measurements with a voltmeter. HOWEVER ... I have no idea what voltage the audio signal might have. If there are peak voltages in excess of 5v they are likely to damage the Arduino so it may be a good idea to use a 5v Zener diode to protect it, until the OP knows what he is dealing with. I have no idea whether the Zener diode can also be the rectifier.
If the OP can get values from analogRead() when the music is playing it would probably be sufficient to assume there is no music if there are no values for (say) 5 seconds.
...R