Induction is one way. You can also do it with electrostatic (capacitive) coupling if both pieces are against the body. I've seen this done and it is quite interesting. You have a modest plate in the transmitter and another in the receiver, both of which are well coupled to the body. The body is the capacitor's dielectric. It probably works best if the audio is modulated onto an RF carrier using FM. I'm not sure what carrier frequency would be best, but starting at 50 MHz might be a good choice. The working implementation that I saw was cool because the transmitter required very little power and there were virtually no electro-magnetic emissions. This was because only the electric field was used, and the transducers avoided generating any magnetic field. There was little power needed because no power is transmitted by electro-magnetic radiation, only electro-static field.