@mitu I have no idea if it's some bizarre edge-case specific to my system, but that's what was broken on my machine and what fixed it - seriously, everything was allcaps CUE and BIN - both the CUE file and the BIN file and the reference to the BIN file embedded within the CUE. All in capitals. I confirmed before renaming that the BIN filename 100% matched the value stored within the FILE "..." BINARY reference in the CUE file.

And I had no music in any of those games.

Changing all the file-extensions to lowercase (including the FILE "..." BINARY reference within the cue file) fixed it.

The USB hub is internal, I can't change it (well, i could maybe). And I don't use that NESpi case, its a different one called NES case, which even don't look like an NES case (its black). Maybe same poor quality issue. I even don't use the fan, because it is loud as a jet. And that pcb is rated to be used with 4 usb connections plus the fan. Not sure if its a power issue.

For now, I will not try more as I don't want mess it up again. I will get a bigger sd card (from 32gb to maybe 128gb, or at least 64gb) and its all done.