All the electronics are mounted on the back of the PECS board. The heart of the whole thing is an Arduino Pro Mini that runs at 8MHZ, so it only needs 3.3V. A DF mini player with micro SD card is responsible for the sound. A TP4056 charging module charges and controls the battery.

The DF mini player has 2 Watt output power, which is quite sufficient. Its TX and RX lines are connected to the Arduino. This allows the microcontroller to retrieve the files directly. 
I have written a small manual for the DF mini here.

Its USB+ and - lines are soldered to the USB connector. This way the MP3's can be written directly from the PC to the SD without having to remove it. At the same time, the battery is charged via the USB connector.
The 8 wires of the matrix are also soldered to the Arduino. A voltage divider on the microcontroller monitors the battery status, and at 3.4V it issues a "Battery Low" warning.