How can I clean speaker holes on aluminum MacBooks?

If you have any of the aluminum MacBooks, you will have perforated areas to the sides of the keyboard to let the sound come through from the speakers underneath. If these holes are dirty, how can I clean them? I have tried with a vacuum cleaner and compressed air, which do not work. ![Upclose photo of the speaker grill on an aluminum MacBook. The photo shows some of the holes filled with a white substance. ][1]