I’ve always wanted to do this, and finally rigged one up! It consists of a 99 neopixel strip from adafruit, a scavenged 3.5mm audio cable, an adruino UNO, alligator clips, jumper wires, a power supply, and an output jack for speakers or headphones. An analog arduino pin is connected directly to the right channel, the grounds are common, and the signal wires just pass through to the output jack uninterrupted. Here’s the circuit:
The gif doesn’t give it justice, neither do the pictures. The color is rich and impossible to photograph, and the falling dot at the top is pushed up by the max level and falls at about 6 pixels/second back down to the current level. It’s very fast and is smooth in transitioning from the different levels. Here it is!
The arduino sketch: