That's always the hardest part to understand. Light would go into it like your eyes, and the two mirrors would have to be tuned to the same wavelength/resonant frequency like a radio. We all know how to do it digitally, but on analog, it's different. The entire mirror itself would need to be constructed in a way that makes it resonate at a certain fundamental frequency, same with the other mirror. Transmission of sound waves/vibration through the air is 100% possible and doable. Ring a loud instrument and hang a resonant object on a string and it will vibrate. Everything vibrates everything but tuning a string to the same frequency as the transmitted sound will cause the string to vibrate even more above the rest.

So both mirrors would have to be constructed/tuned to the same fundamental frequency in order for them to speak to each other, they would probably need resonant cavities like a magnetron. The bad part is you might need to activate the power grid in the town first. This is so the transmitted information through the desired frequency chains with all the resonant buildings, creating a larger range between the two mirrors (amplification through reverberation, like playing a noise in your phone and placing it in a cup). The king would need to have his setup in a resonant building of course, and could potentially build a direct resonant line between the two mirrors (for his non-mobile mirrors) in the form of metal bars/tubes/wires through the walls. Imagine a long device that keeps its resonation over a long distance (ex: tunnels, caves, strings) but a miniature version in the wall. The mobile mirrors would theoretically not be as good as the stationary mirrors for that idea alone. The mobile mirrors would have to only rely on the transmission of the desired frequency through the air and from your body touching the resonant ground of the buildings.

The issue I am running into with all of this is the relationship between sonic waves and electromagnetic waves. All notes have their own frequencies, and all colors have their own frequencies. I need to figure out the relationship between the two and how they work together. Magnetrons were built to resonate both sonically and electromagnetically (they emit microwaves). Magnetrons are found everywhere in churches and cathedrals, which were built to resonate sonically. According to our research on this subject, they most definitely can and have been resonating electromagnetically too. These mirrors work by means of electromagnetism (light going into it), but how does it work sonically and can electromagnetic waves travel like sound waves? That is a serious question I need to answer.