It seems there is a reason why Nokia puts an ugly piece of plastic around the headphone jack. Can you find that on your phone? No? Well, there's your problem!
In Sensation's case, headphones constantly "touch" the back cover, which eventually scratches out the paint, revealing a tiny bit of bare metal. Similar thing happens on (unconfirmed): HTC HD2, HTC Desire HD (reported but unlikely), HTC EVO 3D, Samsung Captivate, Motorola Droid X and possibly a few more phones. Some of these have a silver ring around the jack, but this obviously doesn't help.
So, here it is on the Sensation:
Problems occur once headphones come in contact with bare metal. I tried covering the scratch with paint, nail polish and super glue. None of this works, it just removes more paint. Coating it with more permanent paint would probably work for a while, but that would peel off eventually.
From what I can see, there are two ways to fix this.
One is quite simple:
While two involves modifying the headphones themselves.
It took me a while, but I found a perfect solution:
These are... dental.. rubbers... I actually have no idea what they are really called They are used for braces. I believe you can find them in any dental store. Just bring this picture with you They are under 1$, and come in various colours.
This is what you need to do with them:
Here is another way to do the same thing, done by Mr. Ulmo.
In case this happens on your phone (non-Sensation) and you are able to fix it, please notify me so I can add it here
Also, in case you don't want to do any "hardware alterations", this app blocks headphone buttons, which kind of fixes the problem.
Hope I helped!