I am not against your idea. In the end the hardest parts are going to be automating the winding of the coil and creating/fixing the membrane. So maybe there is

I am not against your idea. In the end the hardest parts are going to be automating the winding of the coil and creating/fixing the membrane. So maybe there is someone out there who enjoys going through a ton of prototypes to get something reliable out. But the issue I think this is going to have is how reproducable that is going to be for your followers. I think in open source there is a spectrum: one the one side you have things like most software or simple standard-parts open hardware projects. On the other side you have things like an open source marble statue, where you still need to bring all the knowledge of how to make the marble statue — draw a circle, draw the rest of the owl and all that. A microphone with complex coil winding and gluing a micron-thin membrane to a coil is going to be somewhere inbetween. What exists btw. are condensor mics, but there you need to buy the membrane-assembly yourself.