Replacing electret microphone with audio input?

[Resistanceisfutile]

I have a device that uses an electret microphone, but I wanted to send audio from my MP3 player into the circuit instead. Can anyone tell me whether I should just remove the microphone and wire the ends to my audio source wires, or would I need to go further into the circuit to do this?

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My source is mono if that makes any difference.

 

[Audioguru]

Some electret mics use two wires and others use 3 wires.
A resistor powers the Jfet inside a two-wires electret mic and its current might mess up your MP3 player. If you can't remove the resistor then use a 330nF film coupling capacitor.
The output from the mic is mono at about 10mV. The output from an MP3 player is stereo at about 270mV so a stereo to mono attenuator is needed like this: