“Historically, sound has been used in everything from animal communication to computer-human interfaces to warn us that something bad is about to happen: a loud sound warns you that you’re about to be squashed by a garbage truck, for example. This may seem obvious, but it’s central to the discussion of audio feedback in any interface. Though they’re not life-threatening warnings, the sounds a product makes are there to contribute to its usability, enjoyment, and brand identity—in some cases in more compelling ways than its form or functionality.”
The article contains a very good history of sound design and mentions some of the current design constraints. For instance, the tiny plastic speakers, so prevalent in current consumer products, are much better suited to emitting the familiar high-pitched chirps and beeps that make up the modern vocabulary of digital devices.
The bulk of the article is on what designers should be focusing on when dealing with sound: legibility, context and usability.