At a very simple level, if something is not looked at, then it cannot be seen. However, if something is not listened to, it will still be heard. This is because hearing is not directional. Hearing is the “process, function, or power of perceiving sound; specifically: the special sense by which noises and tones are received as stimuli.” Taking this further, hearing is passive, involuntary, effortless. Yet it can still have powerful effects.
Details at warc.com.