Hi!

You know that wires are coming out from mp3 player's 3.5mm jack to the two speakers of the stereo earphone.
I want cut that wire of the earphone and take wires to input to a FM modulator as the modulating (message) signal. Is this possible?

Simply I want to do something similar done here to test a FM transmitter. http://people.virginia.edu/~ll2bf/docs/various/fm_xmtr.html

I want to do it just by using the earphone connected to the mp3 player.

And how I identify the wires (ground, left, right) properly and how I make the stereo signal to a mono signal?

 

The transmitter circuit has two resistors R3 and R4 connected to its audio input jack that combines the two stereo channels into mono.

You can fiddle around with the wires in an earphone or you can buy a 3.5mm plug to 3.5mm plug stereo cable. The earphone cable might pickup mains hum or other interference. The stereo cable is shielded so it does not pickup hum or interference.

The simple transmitter is missing pre-emphasis (treble boost) that all FM radio stations have. Then the matching de-emphasis (treble cut) in all FM radios (to reduce hiss) will make the sound muffled like your stereo with its treble tone control turned all the way down.