How to play a microphone out of a speaker?
The Op's question reminds of a tricky question paper :) . What I construde is; 


1) 3.5mm Jack= O/P (looks like an I/p).
2) He wants to insert electret microphone into this jack .


But If he is listening to something with MP3 then the Jack is not an output device , he probably has an additional speaker somewhere ?? .If this is true then Jack is an I/P connector. The electret mic cannot be inserted directly, you need biasing as below .


It would be better to add a pre-amplification stage as below;


from: here

When you plug into the microphone , slowly adjust the gain of your opamp and O/P volume of your amplifier such that feedback is eleminated.