Bar speaker distorshion?

My Philips sound bar keeps getting a distorted scatchy noise whenever the voices talk at a sertant tone but the weird thing is that it comes an gos and when it comes I just have to move the optic cable uh lil and it sounds better for a lil bit or it gos away on its own, either way it comes back. Anyone can tell me wats going on and can radio shack fix it? If they can hoe much dos it cost