It's a month later, but hell I'll give a guide on this one.
Recently i had the same problem, but a bit worse, a part of the cable of my 457 was torn (at the point where it enters the plastic cover).
Anyway, how to solve it:
Step1: Remove the broken earpiece from the cascet (kinda logic), pretty simple, just slide it off the cascet, it can happen that u have to use a little force.
Step2: Removing the fabric cover (the black cusion) from the earpiece. It's locked on it with clips, so open it on 1 side (use your fingers and a BIT of force to find an opening), then using a flat object, like a screwdriver, go around the and crank the opening a bit more open (-->so it's, find opening, put screwdriver between it, go a bit further with the screwdriver, find another clip, force it open, go further) and before you know it it's open, remember only use a bit of force, don't break it open.
Step3: there are 4 black screws in the next plastic layer of te cascet, unscrew them and remove them.
Now you see the open cascet with the wire going into the real headphone, don't touch the magnet (the middle round thing). As you can see the wire splits in a red and green wire which goes into the solded part of the real headphone.
Step4: Most of the time the wire get's cracked on the point where it meets the cascet, not the inner parts. So what you have to do is a bit of wireslaying. Cut the wire at the big plastic part outside the cascet and leave the long cable for now. With the cascet, you'll be needing some force. Use a sharp object to cut the big black plastic outside the cascet at the points where it meets the cascet. Cut it until you get to the metallic part. Then go around with the sharp object till it's completely cut to the metal part. Then remove the black plastic and on the other side pull the wire out of the hole. It could be that the wire is still difficult to be removed, because it's locked with a metal plate and black plastic. Then you have to Carefully break that black plastic part with a knife/sharp object until u can pul out the wire (normally ypu don't have to do that).
Step5: Get the solding equipment (aka solding jelly, solding material (metallike wire) and soldation machine. Now I don't exactly know how to do the solding part right. But i just used the machine on the solded part of the wire. Anyway, be REALLY careful with the solding, don't do too much preasure on it , just heat the soldation material on the headphone/wire part and remove the wires (red and green). You can eventually use a pincet to carefully take out the wire and remember, use this carefully since the machine is heated and could damage the earphones when you hit a delicate part of the headphones with it.
Step6: Now u can do anything you want with the little piece of wire, you can also use it as a reference for the solding you'll be doing. Now take the cut-off long cable you'll be solding and cut of around 1 - 1.5 cm of the surrounding plastic. Next bend the the wires (actually they are made from a lot of little wires) and if they are a bit "open" roll them back to a nice wire (many little wires, roll them with your finger until you made 1 red and 1 green wire).
Step7: Now get the solding material, begin to heat up your solding machine and put some liquidated material on one of the solded parts of the headphone and quickly put the correct wire in it (soldation material quickly melts/hardens), also I would recommend using the green wire first and then the red wire, also use the TIP of the wires). If they are solded solid into the headphone, test it out. If you got the wires right, the phones should work as they normally should. Otherwise make the solding material which holds the wire liquid again and reput the wire in it. You can also test the wires by trying to put them on the solding parts of the phine (so with your fingers you hold the wires on the right part, could need some effort to work).
Step8: Put the plastic lid back on, screw tight and put the fabric protector back on the plastic lid. Put the phone back on the cascet and voila, you're done. It could be that you might adjust the splitsection of the wire a bit, because the one wire is offcourse smaller now.
Hope this Could help some people
