In a manner which is not capable of being heard or not loud enough to be heard.(adverb)
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Use "inaudibly" in a sentence
"Where's my wife? he muttered almost inaudibly, his face buried in the pillow as she cleaned his waste from underneath him."
"It was as though she were asking me, inaudibly, “Feeling a bit jealous, kid?”"
"There's an onstage jazz trio Peter Erskine, John Parricelli and John Paul Jones, no less who appear briefly and mostly inaudibly in a single short scene."