I just learnt about such a thing as a crystal radio receiver and the following idea came to my mind. I have no background knowledge on electronics nor anything related.
What if battery-less earbuds can be made, picking AM waves from very short range nearby stations such as an antenna plugged in the audio-out of my pc or smartphone or laptop or all 3 of them at the same time, the receiver shall mix them together in this case and differentiate left-right channels for stereo. Not sure if AM would suffice for the 20kHz bandwidth necessary to cover the full audio spectrum but FM would be more complex for little benefit i think?
The antenna would be inside the string that connects the two earbuds for maybe a max of half a meter length. If two antennas (one per earphone) are needed they can both coexist in the same string but it would be thicker.
This would be compatible with any standard audio plug and have enough range for an apartment.