Hey everyone, I have a Yamaha P105 keyboard that I picked up for free. It's in great physical condition, but the sound is...

Hey everyone, This is not a question specific to this Macbook, but I am just trying to understand how this works all...

Well, I managed to fix it, and just posting here in case anyone comes across something like this in the future. The service manual was extremely helpful in finding the part of the board that was damaged. Upon further inspection, it looks like there was some paint(?) that fell onto the board somehow and corroded the traces ever so slightly that they ended up detaching from the resistor networks. I scratched off the solder mask to check for continuity, removed the resistor array, and repaired the traces with a single strand of 30 AWG wire. Everything works great now, but this certainly was a long troubleshooting process. Thanks again for the help @jayeff !

Thanks for the reply @jayeff, the service manual is certainly the most useful resource for sure! Looking at it, page 61 is a great starting point, I need to start measuring the values once I get a meter and a scope hooked up to it. Interestingly enough, the chart on page 61 shows that IC1 (the main processor) is the culprit if the Speaker Output, Phones Output, Aux Output and DAC signals are bad. If all of these is bad except for the DAC signal, then it could be the "Error Detect Circuit Drive", which is very interesting. I have a hard time understanding how the main processor is bad if everything else works, but I need to start probing around. I will report back what I find.