Something audible can be heard. Sometimes things that people don't want others to hear become audible, such as a growling stomach during a quiet moment at the movies.
Coming from the Latin audīre, "to hear," audible is an adjective meaning "heard" or "hearable." A scream is audible, a barking dog next door is audible, and a siren is audible. Why do some writers use the expression "a barely audible whisper?" Isn’t that what a whisper is — something you can barely hear?