The hardware is a collection of modules on a custom PCB, aside from the ESP32 there’s an I2S microphone and I2S audio amplifier, which along with battery and speaker are housed in a neat 3D ...

Part of what makes this all work is I2S (Inter-IC Sound), a format for communicating PCM audio data between devices. Besides the ESP32, at the heart of it all is an SD card reader breakout board ...