Good evening from the Philippines! Would you kindly tell me the correct verb for 'audience' in this context?

The audience (is, are) seated at different levels in the auditorium.

Many, many thanks!
Either is possible, daddyjohn. Much depends on whether context conceives or presents the audience as a single body or as a large number of listeners:

The audience are clapping their hands madly.
The audience is quiet.

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Thank you very much again, Mister Micawber.
And if we need to substitude the word "audience" (or public) by a pronoun, will it be "it" or "they"?

If you are going to need a pronoun, then cast 'audience' in the plural and use 'they'.
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She rounded out her speech with a powerful call to action. It brought the entire audience to its feet.