Use a meter to check the wires from the plug to the speakers. See if one has no continuity (probably the right pos wire). Most likely there's a break where the wire enters the headphones or crosses sides- where the wire is flexed most often. You won't probably see it. You'll just have to replace it or cut the existing wire shorter and rewire them. If you don't have continuity issues, you could have a bad earpiece, but I doubt it.