Keep in mind, sympathetic vibrations are very weak in comparison to grounded vibrations (the emitter and receiver being connected physically). Using sympathetic vibrations as a form of communication (the black mirror, as mentioned above) would require a more advanced setup. We know from experience (put your phone in a glass and play music, or yell in a tunnel) that passive amplification (amplification without the need of external power) is very possible. We just need a setup that satisfies our goals. Here are my theoretical specifications:

The Transmitter
In this case, the transmitter is the first black mirror. It receives light from the front, transmits it with its electromagnetic (and sonic) resonators, being internal magnetrons, into the air/ground (if mobile or stationary as mentioned above) contacting the passive amplifier.

The Amplifier
A number of various objects could be the amplifier. In this case, it is the resonant buildings and its insides (resonant walls, pillars, room cavities, and magnetron cavities). The signal from the transmitter is sent into these resonators (passive amplifiers) and becomes much larger. The level depending on the magnitude of the amplifiers used. The level dictates the communicational range between the transmitter and the receiver, so its probably not going to reach outside the village unless your oligarch family set up the sickest underground resonant pathing system in between the neighboring villages.

The Receiver
In this case, the receiver is the second black mirror. Whether this mirror is exactly the same as the transmitter, just in a different configuration, I do not know. Regardless, this is the mirror you see out of into the other side. From the amplifiers, the signal gets sent into the receiver. The medium of transportation is dependent on whether you have a grounded or mobile mirror. The mobile mirror should probably have a setup where you can ground it in places, but for the purpose of this discussion, we will assume you are holding it in your hand. The mobile mirror gets the signal from the air (grounded mirror from the ground) emitted from the amplifiers which the mirror then electromagnetically interprets the signal as visible light, which is the emission of light from the energy it gets from the amplifiers (if on mobile setup, that is sympathetic vibrations as said above). Also worth noting, the mobile setup (black mirror in your hand) is not completely powered by sympathetic vibrations because your feet are the ground. Since human bone vibrations (which is dampened by the flesh) is not nearly as strong as palace/cathedral vibrations, sympathetic vibrations may have to be exceptional in order for a mobile hand-held black mirror to function accordingly.

We need evidence that magnetrons can speak to each other similarly to strings speaking to each other. Essentially, if magnetrons can vibrate sympathetically electromagnetically, then that means it is possible to transmit an image MID-AIR and in all directions with them in the form of microwaves. At the very least, we can convert it into an audio signal like a radio wave.

If it is possible to convert visible light into microwaves and back into visible light again, The Black Mirror is very possible.