Those cracks are not going to have any impact on how they sound. It is more a matter of visual appeal and comfort. If the look bothers you or you can feel the cracks on your skin, then consider getting replacements from Amazon. That's what I did for my daughter's Senheisers. She said they were a little difficult to remove but she managed to replace them herself all the same. I ordered a new chord at the same time to save on shipping as she wears them daily at work and I knew it was only a matter of time before the chord would wear out.
So it's a personal choice. My daughter had her headphone pads covered in duct tape to hold them together and after a while it almost became a badge of honour.
They would probably still be taped together if I hadn't sent her replacements.