Sort of sound a dry speaker cant use? (6)

I believe the answer is:

liquid

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'sort of sound' is the definition.

I cannot understand how the rest of the clue works.

(Other definitions for liquid that I've seen before include "Non-gaseous fluid" , "Sloppy" , "Melted" , "Neither gas nor solid" , "substance in a fluid state" .)

I've seen this clue in The Irish News.

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