A simple demo demonstrating the use of two undocumented Gameboy Color registers, nicknamed PCM12 (FF76) and PCM34 (FF77), which can be used to read the current PCM amplitude of the 4 APU channels. The demo uses these registers to visualize music in an oscilloscope-like manner. The oscillation wave is a bit of unusual, because a Gameboy's waveforms are not centered.
GBVisualizer was designed to work on and tested on a Gameboy Color. PCM12 and PCM34 are not likely to exist on an original Gameboy, but I do not have one to verify. As of April 2017, the only publicly released emulator that emulates these two registers and supports running this demo is SameBoy (GitHub).
Music for this demo is adapted from my disassembly of Super Bomberman.