All iOS devices have headphone jacks and all but the very oldest have speakers as well. The same is true of microphones. The SDK provides ways to capture and play back audio.
The microphones and speakers are also used for accessibility features such as the VoiceOver screen reader. You can build descriptions into your graphical user interface (GUI) elements to enable your applications to take advantage of VoiceOver, so your interfaces can describe themselves to visually impaired end users.