This have nothing to do with fvwm but is just something I made.
A theremin is one of the oldest electronic music instrument. https://en.wikipedia.org/wiki/Theremin
To play it, you have 2 antennas and the movements of yours hands near them will change the sound. With one antenna, you vary the volume, with the other one, you vary the tone of the sound.
Into an old vacuum tubes radio, you will find most of the needed parts needed to build a theremin. I converted such a radio into a theremin. At the same time, it makes an excellent guitar amplifier.
It is an unique model because it is the only theremin in the world which have external antennas. I done a website with some pictures, audio files and the schematics, all in the public domain: https://github.com/domichel/black_star_theremin
It work very well. From that development, it is possible to redo it using parts easily available on the market. Doing that, it should be possible to make a print and win a few decibels on the signal/noise ratio of the audio circuits. But well, as it is now, it work already better than my Fender Champ when used as guitar amplifier.
When used as a theremin, the lower frequency of the sound is about 10 Hz, allowing to play very loud sounds, which is really great when you plug its output into a main and powerful sound installation.