Have you ever known that a package was going to be delivered before the delivery person rang your doorbell? Can you tell the difference between hot and cold water being poured into a glass? How do you know which of the screaming children on the playground is yours?
People use sound as a means of understanding the world in a huge variety of ways. Sometimes these are fully conscious: you've learned the sound of your child's voice and are actively listening for it. Sometimes the cues are more subtle: you've gotten used to the sound of the UPS driver pulling up in front of your house, honking, and setting the warning flashers on the truck. And sometimes, the sonic cues are too subtle to even be aware of: hot water has more energy and therefore produces a higher pitch when poured.
In all of these cases, sound is a critical component of someone's experience. And if you want to effectively design for that person, you need to consider sound as part of the overall ecosystem in which your design is going to operate.
Soundscapes and Sound Design
When you're outdoors, you're surrounded by sound. If you want to use your phone, you need to be able to hear it above all the ambient noise around you. So when designers are thinking about how to make a phone that can work outside, they need to consider the soundscapes in which the phone will be used.
Like other elements of design, sound needs to be treated as part of an ecosystem. If you're designing household appliances, you might want to create sounds to notify owners when something important happens (like the end of a wash cycle). When making a mobile app, you build in sounds to alert the user when there is a change of the app's state (such as a "new mail" notification).
There are much more serious situations with more severe consequences for bad design, such as an airplane cockpit that plays too many alerts at once, or a medical device that needs to get the attention of a surgeon who may be in the middle of a life-or-death operation.
Bad sound design in your UX or UI can create an irritating or unpleasant experience for your users, and, in the worst cases, can literally lead to injuries or deaths. To responsibly design a sonic UX, you need to think very deliberately about how people perceive sounds in contexts. Sound plays a major role in shaping how people behave and perceive the world, so it should be treated as an integral component of your designs.