So, I handed the belt synth to my friend Brandon to see if he could figure out the strange audio issue with the LED making noise in the speaker.
His solution was to isolate the audio section ground from the rest of the circuit.
You can do this by cutting the traces on the GROUND (battery) side of C1 on both sides of the board so that they don't connect anything, and then running a bodge wire to the exposed conductive pad at the back of the amplifier chip (the exposed rectangle with the 6 vias on the right of the board, when you are looking at the front.)
You also cut the connection between C4 and GROUND (battery) so that the only connection is to the back of the potentiometer terminal 1.
It seems odd, because the ground at the amplifier is still connected to the battery by a longer route, but somehow this seems to work.
I have a nice picture of what to do, but I keep getting "Unknown Error" trying to upload it, so I can't show it.
Anyway, I've updated the KiCAD schematics and PCB on my system, so I'll probably try to test them before making them public.
I wonder how to specify the bodge wire in the PCB tho?
Does anybody know what the best way to cut traces on a PCB is? I have ~60 to correct...