Didn't accept it should be muted (6,4)

I believe the answer is:

turned down

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'muted' is the definition.
The answer and definition can be both related to change as well as being past participle verbs.
Perhaps you can see a link between them that I don't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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