Finally after a long wait, I can confirm that the board received MIDI signals. The board then transforms the MIDI note it received into values for the registers of the YM2149.
The MIDI communication has been made possible thanks to the MIDI.h library found here. Although I have modified it so that it only receives data. The modified version is in the github repo.
Finally, the onyl commands that I have implemented are the NoteOn and NoteOff commands, However, I would like to implement some code to be able to treat the velocity. This would make the board more "natural" and not just On Off.
That's all for now :D