Volume of moans gently audible (4)

I believe the answer is:

size

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'volume' is the definition.
(I've seen this in another clue)

I don't understand how the remainder of the clue works.

(Other definitions for size that I've seen before include "Amount" , "Dimensions, magnitude" , "Measurement or extent of something" , "Volume" , "Measure" .)

I've seen this clue in The Sunday Times.

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