Those grates actually don’t cover the speakers. The speakers are located underneath your keyboard. With that being said, it looks to me as if you have something stuck inside those grates rather than a coating removed. Unfortunately Apple couldn’t actually replace anything as it is cosmetic and they would not cover that under AppleCare or the included warranty. Perhaps if you have some type of adhesive tape or something similar you could remove whatever substance has found it’s way into your grates.