And now my system is constantly dropping the USB connection, and I just saw a spark come out the USB C port on the Mixer when plugging it back in.
WTF?!
Somehow, I missed this post. That means there is an electrical potential difference between the PC and the M12. That definitely could be causing audio issues. Sparks mean there is a current flow. Noise riding on the current could be introduced into the audio that's passing between devices, even in the digital domain. Troubleshooting these kinds of things can be daunting if you're not up to speed on grounding theory, but don't let that throw you. Just be methodical.
I would start by disconnecting every outboard device from both the PC and M12. Then, connect the PC and M12 together and use a mains ground-lift plug on both the PC and M12 and see what happens. Sometimes, only 1 device should be grounded at the mains and sometimes both - and sometimes
neither. Also, you can try to add a grounding wire between the devices. Add devices to the system (i.e. plug in your keyboards, outboard gear, etc.) 1 at a time and observe the results. Apply ground-lifting to each and observe the results.
Don't assume anything. For example, my previous studio computer
had to be ground lifted or else I would hear noise everywhere. My brand new studio computer is grounded at the mains and my audio is
silent everywhere.
Back in the early 90s, our studio was developing a high noise floor. I took a long piece of welding cable and brought it up from the street side of the water main, up through the floor of the studio and to the big copper plate in the patchbay section of the console which was specifically designed to be the studio "star" ground. As I brough the cable terminal to the plate, there was a spark and a horrendous noise appeared in the studio monitors. Somewhere, something in the studio was passing current and that usually means a bad cap in a power supply. One by one, I ground-lifted all the outboard gear - delays, compressors tape machines, etc.. Of course, it was the very last piece of gear at the far end of the room - a vintage 2 track Scully machine. As soon as I pulled the power plug, the noise went away. The noise floor dropped to less than -70dB - excellent for an analog room. Sure enough, one of the power supply caps in the tape machine power supply was leaky.
In my personal studio, I bought all new USB cables with inline chokes (those big round molded things near the ends) and I even added a few snap-on chokes to them. After going through several troubleshooting sessions over the last 3 years, everything is whisper quiet and no audio glitches.
