The media capabilities of our mobile devices are frankly amazing, especially when you consider where we were five, ten, fifteen years ago. The first mass-produced MP3 player was the SaeHan/Eiger MPMan (http://en.wikipedia.org/wiki/Portable_media_player#SaeHan.2FEiger_MPMan) introduced in 1997. The device had 32 MB of storage, enough for roughly 6 to 7 songs (assuming 1 MB/minute, 5m per song). While it may seem paltry by today's standards, it was a revolution and spawned a new way to listen to music.
Today's devices are now so much more, portable entertainment devices that can play games, video, and all sorts of audio. Being able to play sounds in your app is critical, and there are few apps that could make the case for having no sound whatsoever. Although a bit on the extreme side, perhaps, TweetBot is a classic example of an app that is enhanced by the sound it produces via the user's interactions.
Today's devices can also record audio for a variety of reasons...