amundsen:
By the way, I was thinking about heterodyning too but actually is it something different from amplitude modulation ?
Yes it is frequency shifting not a form of modulation at all.
amundsen:
It might be easier but the continuous nature of an audio input sampled by a steady and high-sampling rate clock makes it much more reliable for sensors IMHO.
I don't think you are right. What sort of sensors are these?