Hello @Kombistack
With Arduino IDE, Core2 AWS and when I use Port C Blue the examples works.
You need to change RX 16 to 13 and TX 17 to 14 in the INO sketch :
synth.begin(&Serial2, UNIT_SYNTH_BAUD, 13, 14);
And in the file "M5UnitSynth.h":
void begin(HardwareSerial *serial = &Serial2, int baud = UNIT_SYNTH_BAUD,
uint8_t RX = 13, uint8_t TX = 14);
Happy Midi ;-)