it's still getting more strange. Either we have some misunderstanding or things are really weird on your board. To prevent the former let me just summarize what we did and what the results were. I have attached a picture of the circuit we're dealing with. As you can see this is nothing fancy.
At the beginning you removed R70 to disconnect the digital volume control. After that it's just the pot, a bit of RC filtering and the input pin of the TDA. For clarification I have named the input side of R76 (the pin closer to the rim of the board) as 'Node A' and the other pin of R76 (which is connected to pin 3 of the TDA) as 'Node B'.
Our goal is to use the pot to control the voltage at Node B, but for some reason you can only control the voltage at Node A, but not at B.
At first you checked R76 to ensure that the electrical connection between Node A and B is OK. You used a multimeter to measure the resistance between A and B and found it to be in the range of 300K which is fine. You even measured this resistance between pin 3 of the TDA and pin 2 of J4 to check the corresponding board traces also.
As the voltage at Node B was different from Node A and R76 was fine, it had to be a current through R76 causing a voltage drop at this resistor. But this current has to come from somewhere so we tried to exclude the possible candidates. First you lifted pin 3 of the TDA and after this didn't help you also removed C10.
However, when I got you right then all of this didn't help, the voltages at Node A and Node B are still not equal, right?
That means we still have a voltage across a resistor and according to ohms law this requires a current. But since you have removed all electrical connections except the resistor itself there should be no path left for the current to use and that means there has to be an unwanted current path from Node B to somewhere else.
Please check the pads of R76, C10 and pin3 for possible shorts. It might be a barely visible droplet of solder, so try to use a magnifier. And be sure to check the pad of pin 3 also on the top side of the board, may be something is hidden there. Try to measure the resistance between Node B and it's neighbours as well as GND, +5V. None of these values should be less than the value of R76, because this should be the only way out for the current.
Even though it is extremely unlikely, it might also be a board manufacturing problem. We know, that the design and layout is working, because several people are using it already, but it might be an issue with your particular board. Therefore please take a close look at the board traces also.
I'm afraid that's all I can do for you from the technical point of view. If all of this doesn't help we can try to setup a Skype meeting to look together.