In vain did Elizabeth endeavour to check the rapidity of her mother’s words, or persuade her to describe her felicity in a less audible whisper; for, to her inexpressible vexation, she could perceive that the chief of it was overheard by Mr. Darcy, who sat opposite to them.
There are no more uses of "audible" in the book.
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She spoke in a barely audible whisper.
It was so quiet, our breathing was the only thing audible in the room.