Change volume on song and one cuts quality of sound, almost (9)

I believe the answer is:

variation

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'change' is the definition.
(variation is a kind of change)

I can't explain the remainder of the clue.

(Other definitions for variation that I've seen before include "Change, difference" , "Slight change" , "Discrepancy" , "Slight difference" , "Departure from the norm" .)

I've seen this clue in The Telegraph.

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