Re: Data trasmission !!
You can probably send the data as a 1 bit digital word. +50 mV for a "1", -50 mV for a "0". The problem will be if the data rate is too slow or too fast. The microphone input is probably AC coupled, so if you send data at a rate slower than perhaps 200 bits per second, it will not come out like a square wave at the other end. If you send data at over 10 Kbits per second, the 15 KHz lowpass filter in the FM receiver will start giving you trouble.
At the receiver end, a simple ac coupled comparator op amp can recontruct the digital word.