alt Hacker NewsAmateurs in USSR 50 years ago made wireless and powerless headphones, which use wire lay on perimeter of room to transfer sound and power.
In headphones there is tiny coil.
It really work and very reliable, but result coil (size of room) have very large reactive resistance, so it is nearly impossible to transfer even high frequencies, only low (bass) and medium, so it workable for speech but music is heavily distorted.
they also made microphones work like this. place em behind the panels of sockets on the wires. some accounts of such devices being found can be found in the book Spy Catchers (by some ex MI5 science officer during cold war). pretty interesting tech!