The microphone needs a preamp circuit with a voltage gain of about 30.
Then you can use one jack on the input of your circuit for the output of the mic preamp and use a second jack for the output of your MP3 player.
Use a switch to switch between the two inputs or make an audio mixer circuit.
Then you can use one jack on the input of your circuit for the output of the mic preamp and use a second jack for the output of your MP3 player.
Use a switch to switch between the two inputs or make an audio mixer circuit.