Now three Core modules are assembled apart from the 3.5mm jacks. The Xtend module did not work, I had switched VCC and GND for the OLED and probably missed other things as well when designing the PCB.
So I decided to skip the Maple Mini and use the PCBs I have for the Core modules. The audio I/O jack can easily be replaced by two more UART connections (TX/RX2 & TX/RX3). So the XTEND module be similar to the CORE modules except the audio I/O will the replaced by two more data I/O=5 UARTS!
CORE is now SONIC.
XTEND is now SIGNAL.