The natural world sends us signals all the time. Birds and squirrels let us know when we're transgressing their territory. Clouds gathering overhead tells us that it's probably going to rain. Because they're natural, a lot of these signals are pleasant to us. Biophilia is the human love of living systems, and one of those ways that systems let us know they're alive is by sending us their signals.
One of the hardest aspects of design for software is that we're building interfaces that have no natural feedback, so we are tasked with determining what the natural feedback of an action might be. The current array of sounds that our devices produce are overwhelmingly non-natural. When half of our appliances beep relentlessly to signal something's finished or done they aren't uniquely identifiable and we're left wondering what's actually happening.
Yet there are exceptions. The iPhone's "text message sent" sound is unnatural but it's satisfying, and somehow the perfect sound for what it implies. It's the sound I imagine those pneumatic tubes made in the late 19th century - something I have no actual reference for.