Yesterday, I liberated the Atmega328p from the Arduino Uno, and installed it in the breadboard. I rewired the full thing to fit on the one board, changed the resistances for the LEDs around to 220Ohm (as I moved from the 9V to a 3V power source), and had to fiddle with the code a bit to get the ports correct. I’m pretty pleased with a fully working breadboard prototype. The next step is to solder it to some perfboard, and have a mobile prototype ready. Perhaps if I enjoy it enough, I’ll actually make some PCBs for this.
One thing that I might consider to add is a small switch near the buzzer to turn the audio off or on if needed.