"Sound" refers to a type of wave that emits from particles when they vibrate. Humans can hear only a certain range.
What you hear at any given moment depends on how your brain decides to process auditory information. Your brain is constantly making "decisions" about what details to bring to your conscious awareness. The details that you become aware of are within your range of perception. The details that you remain unaware of are outside your range of perception.
The brain discerns what sounds you need to consciously hear, and what sounds you don't. For example, non-important background noises are usually not registered by the consciousness. That's because so-called "white noise" isn't an aspect of your environment that you need to be aware of. It's like the sensation of clothes against your skin. You may feel that sensation for a few moments after you put on your clothes, but throughout the day you generally won't feel it unless something changes. Likewise, with "white noise," you'll only notice it if something changes: e.g. the air conditioner suddenly hums louder, or the dryer stops running.
If you perceive danger or if you're in a life-threatening situation, you'll hear things you otherwise would not. You'll be attuned to many more details in your environment because your brain wants you to be aware of any detail that might help you to protect yourself.