Son's headphones don't put out any sound on left earpiece but I get continuity from the circuit board to the four solder pads on the speaker. This surprised me as I thought it would be a classic case of the broken wire syndrome. 

But I am also puzzled by the fact that the wires are 'bird-nested' so wonder how they are insulated. Are they coated with an insulating layer? Guess they must be.

Any tips on fixes or troubleshooting welcomed.