You should consider, that a standard headphone has an electrodynamical voice coil actuator, that has the magnetical field mostly shielded. Any coil with sufficient number of turns can sense the field.
So if I were to make it from scratch I could just wind the thinnest copper wire possible around a the strongest pole magnet as many times as possible and then connect each end to an amplifier circuit as in the link KAK posted above? or do i not need the magnet and I just make a coil?