Microphones might be the most important symbol in this movie, because they symbolize the transition from the old into the new world of mass media. Through the microphone, the speeches of kings are able to reach every citizen in the British Empire. In the past, most British subjects would have never heard the King's voice. Or as King George V puts it,

KING GEORGE V: This devilish device will change everything if you won't. In the past all a King had to do was look respectable in uniform and not fall off his horse.

In the old days, Bertie could have gone his whole life as king without worrying much about his stutter. But in the modern world, he has to give speeches into microphones and he can feel the whole world judging him. His handlers keep telling him,

HANDLER: Let the microphone do the work, sir.

But that's just the problem; Bertie is terrified of the microphone and all the silent judgment it symbolizes.