Follow-up: still no sound.
Koma unfortunately didn’t have a USBtiny, so I started by replacing the chips instead. The nearby electronics store only had the LM386 part, so I replaced it, but still no sound.
Maybe the new chip overheated while I was soldering it in (it got painfully hot at one point), maybe the board is damaged from removing the old chip (the PCB rings look a bit messy now), maybe the 6N138 is damaged, or maybe the software needs to be reflashed. I did recheck all the components were in the right spots and looked OK so I think its one of those issues.
Next week I’ll order a USBtiny from your store and a 6N138 and see from there.
Do you have software for a MacOS ?