If a sound is inaudible, it is too quiet to hear. Your ears get nothing you can use — or so little that you cannot make the words or the noise out. A whisper across a playground can be inaudible; a voice behind a thick door can be inaudible; some high sounds are inaudible to people but not to dogs. We also stretch it a little, and that is how it is really used: “almost inaudible” means you could barely catch it, not that it was truly zero. The word is about hearing, not about understanding. If you hear every syllable but still cannot follow the idea, that is a different problem. The opposite is audible: loud enough to hear. Inaudible is ordinary in writing and in careful speech. Though actually, when we talk we often skip it and just say “I can’t hear you” or “speak up.” It is not a rude word; it is a fact about sound.