The orchestra was Prince Prospero's way to bring entertainment into the abbey. They really set the mood for the story since everytime the clock would chime, the music would stop and everyone in the abbey would go silent to listen to the chimes of the clock.
"Taking hue from the rooms, and causing the wild music of the orchestra to seem as the echo of their steps. And, anon, there strikes the ebony clock which stands in the hall of the velvet. And then, for a moment, all is still, and all is silent save the voice of the clock. "
The clock symbolizes the time for the people inside of the abbey. When the clock hit midnight, it was to symbolize that their time ran out.
"But these other apartments were densely crowded, and in them beat feverishly the heart of life. And the revel went whirlingly on, until at length there commenced the sound of midnight upon the clock. And the music ceased, as I have told; and the evolutions of the waltzes were quieted. "
"Prince Prospero was happy and dauntless and sagacious." The prince had "eccentric yet disgust taste."