If these mirror don't require any physical connection, and yet they are able to transmit light and sound in perfect fidelity from one point to another at the speed of light, it is very likely that they can do even more than this.
You have no idea how it works, but are able to follow simple instructions which it sounds like anybody could do to build the mirror, so the principles involved appear to be fairly simple. Specifically, it's not like a building a large computer where you need a billion dollar semi-conductor fab to build the wafers, it's more like the first transistor where you can build one out of materials that you are able to obtain. Depending on how magic works in your world, it may be that like building a transistor radio that takes up a kitchen table, there is a 50 year or more technological growth curve where this sufficiently advanced technology can be studied and refined. Or alternately, magic in your world may be totally separate from physics and follow its own rules or logic.
Either way, the value is this is potentially incalculable. If you are willing to put the time and effort into it yourself, you could raise venture capital and lead the effort to research how the mirror works and additional effects that are possible. If you can send light, what other things can you send? Can you send gravity? If you can send sound, can you send other kinds of physical vibrations? Can you send matter? If so that would be of immense value as your magic mirror is now a teleporter. 209
The sorts of research into exactly what sorts of things you can do and what sorts of things you might be able to do with tweaks to the mirror could take years, but also disrupt entire industries. If you don't want to put all the work in, you very likely could make a great deal of money selling the information on how to build the mirrors to a third party - just be sure to put together all these thoughts into what exciting and valuable possibilities could be made available and require that the deal allows you to reap rewards proportional to the advances are made, instead of some fixed sum of money.
Even if your mirrors are only ever capable of sending light and sound, how much light? How much sound? Is it just the visible spectrum or other parts of the electromagnetic spectrum? If there is no attenuation of the signal, then a large mirror was placed in orbit closer to the sun, and a great deal of energy could be beamed directly to the Earth for a variety of uses.
Even if your mirrors actually behave no differently from a camera / screen setup, as the other answers mention you still have what seems to be a more reliable system not requiring wires and possibly even noise free transmission mechanism. What if millions of these mirrors were made, but microscopic in size, you could have a revolution in computers as you eliminate the need for comparatively slow electrical paths. Is there any limit on how many point to point connections can be created at once? It sounds like you may have a better crossbar mechanism as well. The possibilities are endless. Play your cards right and you'll be rich beyond your wildest dreams! 209