I happened to have a few old Japanese made Sony mdr-7506 cans at work ..  and all of them have same broken wire after being used on a daily basis ...   i finally got to find an afternoon to play around and replacing the broken cord ..  

it's kinda hard to find the thing 2-core coated wire they use originally, so i just went with one that came in handy ..   it's quiet straight forward .. by just taking the old one out and putting the new one in through the same path ..    nothing difficult ...   only hack was to drill a bigger hole on the shell, and putting the cone back together requires some time and patients ...   and at the end i secured the wires on the braces just so they longer get cut ..