Remove the screen without breaking it and remove the grill from the inside of the screen and replace with a new one. Could be an expensive job for just a mesh grill. Cheaper option would be take it to a repair shop who has a microscope and ask them to clean it. I clean a lot of iphone ear speakers under a microscope as they are bad for clogging up. Can take a while to clean for the size of it but makes a huge difference to the sound if done correctly.