A cheap enclosure holds all the electronics, left to right: the H-bridge DC motor driver, ESP8266 board, STM32F103 bluepill board, and LM358 amplifier with microphone.

During the last minute I resoldered the microphone so it aligns perfectly with the whole on the enclosure so I would not have to raise the gain. It is quite sensitive but the software waits for 3 beeps spaced 6 seconds apart. It seems quite reliable.