This project can sound pretty complex. There are a few major parts (the button controller, display, etc.) as well as the main MCU. To try and help visual-thinkers see what's going on, I've put this together quickly to give an overview of the project's main components as well as how they communicate:

The colored interconnecting lines show the type of communication as well as the direction(s).

The lines on the sides of the boxes show the power source. The 3.3V regulator pulls it's power from the 5V regulator. The amplifier uses an unregulated rectified power supply from the transformer.