Hello Emmanuel. Yes, handing voices to in-coming notes is something that took me a while to implement on TSynth until I just sat down and wrote a series of operations based on what should happen. It isn't the best way...
Hello, yes probably. If it's the VGND pad where the jack is soldered, used the VGND pad underneath:
https://www.pjrc.com/store/teensy4_audio_back.jpg
You can email the TSynth developer directly for help. He's...