Krdytkyu

The Emperor is annoyed that the crowd routinely chant out of step at the Empire's largest circular amphitheatre. For example, they are supposed to shout phrases in unison such as "Hail to the Emperor, he is great!" and yet they always sound like a rabble.

All powerful though he may be, he realises he can't punish the whole crowd at every event. Bread and circuses and all that.

Nevertheless he decides that just for once on his next birthday his Court Musician must arrange an event where he trains a crowd of people (the chorus) to occupy the front few rows of the arena's seats and chant a spoken birthday ode to him in perfect time.

Question

How does the Court Musician keep everyone in time?

Why does he equip everyone in the chorus with earplugs?

Where is the only place for the Emperor's throne and why?

EDITED to tidy up and make clearer. Apologies if I have changed anything too much.

How does the Court Musician keep everyone in time?

All need to be able to see the Court Musician who will act as a choral conductor

Why does he equip everyone in the chorus with earplugs?

So they don't go off the rhythm, misled by the other singers. Each one of them is surrounded by singers which are at ever increasing distance from them. They here a lot of people singing the same theme but slightly delayed. That's why you can see a lot of singers in pop groups covering one of their ears. This helps them hear their own voice so they don't go off tune and the conductor shows them the tempo.

Where is the only place for the Emperor's throne and why?

All singers must be equidistant so ideally the throne will be in the centre of a circle of singers. This way the sound will travel the same time the Emperor and he will hear them all singing in unison.

Partial Answer:

Why the earplugs?

To minimize the error time when the one's voice reaches another's ear. Another type of signal, perhaps visual, is needed.

I'll post this as an answer to my own question. Please note that @rhsquared got the whole thing right. I'm simply adding a commentary.

It is actually impossible for a full arena of people to sound to any listener in the arena as though the crowd are singing or chanting perfectly in time. Even if they are conducted by someone standing in the middle, the sound delay is always there. As @S.M. points out, even the conductor will be affected by the delay between the back seats and the front ones.

Might be a bit of a weak answer, but

He makes them wear earplugs so that if someone makes a mistake the others won't follow, and even if there aren't any mistakes, the voice of the person next to you will come with a different delay than that of the person standing on the opposite side of the circle; puts the crowd in a circle and sits the emperor in the middle it so he won't hear any delay from the people in the back and it sounds like a "surround" sound instead, all at the same time. About keeping them on time, I don't know, guess the court musician trained them well and we can rely on the quality of his work? Besides since they're all wearing earplugs he can just visually signal them for the tempo.