Again, transmitting audio using VCO?help please?

I 've got task to transmit audio ( voice signal ) using VCO then the output connected to infrared. I still confuse how to demodulate it. Input for receiver (the sensor is photodiode) is Square wave which its frequency change according the amplitude of the voice. If i using F to V, it will convert the base frequency ( F0, i set Fo=100khz, the deviation freq.=+/- 50 khz) of VCO to some voltege too. Then how to get the audio/voice? if using PLL how to do it?
Please give some reference (*.pdf/site/ or other)

thanks
Elits