Inaudibly means too quietly to be heard. The lips moved and nothing arrived.
It attaches to speaking and muttering, so answering inaudibly is a way of describing somebody who did not want to be heard at all. Transcripts of hearings mark gaps with the word inaudible, which is where most people meet it in print. The adjective inaudible describes the sound and the adverb the manner. Its opposite, audible, is used the same way and is far commoner.
1adverbinaudiblyToo quietly to be heard.
“He muttered something inaudibly and looked away.”
From inaudible + -ly.
Etymology from Wiktionary, CC BY-SA