Hi draco,
The -4v is about what I figured. It is supposed to be 0V. So, now we need to figure out what is bad and to do that, you will have to lift up the audio board so you can check a couple of outputs on the last amplifier on the main board on the side the audio pigtail comes in on. Check the last OpAmp and see if it is a LT1112. What is wrong is most likely associated with this opamp.
Before doing that, check the voltage again on the audio board but adjust the gain pot from min to max. The gain pot could also be called the range pot. I think you changed that in your video and it didn't seem to make much difference. So, if this is the case, you will have to measure the outputs of the opamp chip.
On the LT1112, pins 1 and 7 are the outputs. You will need to check both and find out if one or both of the outputs are wrong. Pin 1 can cause pin 7 to read bad if pin 1 is too negative of a voltage. Both should be 0V.
With luck, we will be able to change the opamp and solve the problem. If not, then maybe the offset voltage correction circuit is not functioning correctly. If that is the case, I may be able to fix that also.
Reg