How would you describe Celestia's voice?

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I'd say it sounds like a very good team effort between Nicole Oliver and the pointers given by her voice director.
...
I guess another way I could describe it is... a slightly older variant on Cheerilee's voice?
Hmm... this is difficult for me.

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Maternal? Comforting? Sonorous? Commanding? Elegant?

Angelic.

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She sounds like an amused mother or aunt.

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Angelically looming.

Celestial

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Like how one would describe Cheerilee's voice.

Jokes aside, I would describe her voice as slightly motherly.

Picardian.

I would always want to picture her with a voice that is loving, caring, motherly, kind, dignified, and full of integrity where she would always want to help steer all lost ones in the right direction to help them with their lives rather than what she wants.

Alternates between warm and authoritative. Command presence. Honeyed. Silky.

Beautiful.

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Being voiced by a voice actress.

But to take this in a perspective that is required, I say, as the kind of ruler far too many leaders should've aspired to be like.

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Motherly. ("Playtime is over for you, Discord!")

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KIND,MATERNAL,MAGICAL,CELESTIAL and COMFORTING

Like a sunrise giving a hug.