raschemmel:
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.
I don't see any information about the low state voltage , or even if the signal is TTL (or analog). Without knowing that , how can we answer the OP's question ?
If you bothered to read the original post you would see that the OP asked for ideas.
Thoughts?
If you bothered to read the original post you would see that the OP has indicated that the status of the signal is in doubt.
Logically then the OP is looking for Ideas about how to confirm that his signal is suitable for digital triggering or if he will need to use analog triggering.
Clearly knowledge of the "low state voltage , or even if the signal is TTL (or analog)" is not required in order to respond to the OP.
If you only want to answer questions you should go to stack overflow where the strict question and answer format has rendered that site into one of the most useless sites on the internet.