poor sound or no sound is normally a potentiometer issue its unlikely for it to be R37 Resistor maybe Capacitor C59 but I doubt it .
The fact you shorted the pins and you got sound I would bet money it would be in your interest to replace the Potentiometer as its a friction based component I had the same issue as you with one of the GBAs I bought from Japan and replacing the wheel fixed it instantly.
As for your water damaged one I would recommend taking some bits off it as spares like the quartz clock, shoulder buttons and any chips like the one for Power (U4)
If you can I would take the CPU and RAM as you can buy a Funny playing custom Motherboard that only requires the CPU and RAM to make a brand new one in a custom colour with some handy features.