All that pressure on such a tiny plastic part - for shame, Sony, for shame. What a terrible design.
I bought a replacement swivel part on eBay but once I disassembled the headphones it was clear that unless you cut and splice that wire, and who knows how many tiny little wires it contains, it is for all intents and purposes impossible to remove the broken part and install the new part. That’s when I stumbled across this page.
I fixed my headphones by holding the broken side headphones in an aligned, “neutral” position, and by then drilling two tiny holes through the opposing swiveling parts. It was not too hard to maneuver a small hand drill with a very small 1/16” bit coming at the parts from the earphone side. The holes were drilled parallel to the direction of the headband. I then sanded all the faces with a bit of 100 grit sandpaper, slathered 5-minute epoxy on the faces, and ran a few simple stitches through the holes and the walls of the swiveling parts, and pulled/tied them as tightly as I could, drawing the sanded faces together into the epoxy. Space is tight in there so I used a small curved upholsterer’s needle, a pair of pliers to pull the needle through, and some very strong V-92 sailmaker’s thread that I happened to have lying around. (It’s similar to upholstery thread; not particularly thick but quite strong.)
The epoxy cured quickly as I did this and the stitches provide a little bit of extra structural support fairly near the outer edge of what had been the swiveling parts, so, they’re in a place where they can minimize any twisting effect of the headphones on the repair. On reassembly the faceplate completely covers the stitches, and even though the threads are sticking out a bit they don’t misalign the faceplate at all. Except for the loss of the swiveling function, the repair is invisible except for a thin line of epoxy on the inside of the band.