If a sound is audible, it is loud enough for you to hear. It does not have to be a big, noisy sound. A whisper can be audible in a quiet room; a shout may not be audible if a train is passing. The word is about reaching the ear, not about being nice to hear, and not about being “loud” in the everyday sense. English often says barely audible for a voice, a cry, or a phone buzz that you can only just catch. You will hear it in stories and in careful speech: an audible gasp, an audible sigh, a voice audible at the back of the room. In casual talk, people often skip the word and just say they can or cannot hear it. Though actually, some students first meet Audible as a company name for spoken books; that brand borrowed the word because you listen. American football also stretches it: a quarterback “calls an audible” when he changes the plan out loud at the last moment so teammates can hear. Those uses grew from the same hearing idea, but the home meaning is still “loud enough to hear.”