I’m having the same issue with a Hero5 black. I’ve got it open and testing the red and black wires from battery gives 4.0v, but also the blue and black. The test points across the board are all giving between 1.0-2.0v. Did you ever find out what the problem was with yours?

I am working on exactly the same thing. From what I can tell, the usb mic is powered by +5v DC supplied by the usb cable, whereas a conventional condenser mic would typically be powered by +48v Phantom power (which is technically DC as well). The problem with this is that the +48v and the +5v are supplied in different ways, and the phantom power (needed to boost the signal to a point that it can be heard by the mixing board or amplifier) is too strong for the components of the usb mic. I am currently looking into the possibility of replacing the circuit board but I am not very optimistic. I’m sure it can be done but I’m not sure if it’s worth this much time and effort.