It's basically a Pi, a buck regulator, a tiny COB USB audio interface and an SSD1306-based 128x32 display. Removed all connectors and replaced them with soldered wires so it packs neatly into a tiny box.

The software is all custom and written in C, using Jack for low latency audio and my own driver for the display (Adafruit makes a Python driver but it eats half of your CPU and is not optimized for i2c bandwidth or high framerates).