Bro you're a superstar. That was exactly it.
For anyone else reading this later, I was playing tunes through the janky headphone jack patched to the sound system (lol) initially whilst setting up, installing resolume, etc, and when I unplugged it that output that was automatically assigned disappeared to windows - and also to Resolume, and the dell monitor I'm using has no audio, so primary sound driver, non existant - which hung it up.
Damn super edge case, but had me scratching my head, even a guy using resolume since the first release. Never saw those grey squares before...
Also fyi, this is the first club design I did using Resolume only for pixel bars, no LED walls, no projectors, no NDI, no HDMI, just artnet. And it's killer.