Headphone sound is Muffled/Distant

So my headphones have been sitting for a while. When I try to use them now on any device they sound distant or muffled at full volume. I'm convinced it is an electrical issue I just don't know how to find where the problem is in the wire. I've also tried bending the wire everywhere I can and it seems to be in good shape. Bending it has little to no effect on the sound quality.

Any help is much-appreciated, thanks .