Phonon is the multimedia API for KDE 4. Phonon was created to allow KDE 4 to be independent of any single multimedia framework such as GStreamer or Xine and to provide a stable API for KDE 4's lifetime. It was done to fix problems of frameworks becoming unmaintained, API instability, and to create a simple multimedia API.
For example, a file can be played in four lines of C++ code, compared to 30 lines in the old audio framework (aRts)
Phonon is not Unix-specific, and backends can be written for it in order to provide the same functionality on other platforms such as Microsoft Windows.
Phonon is not designed to have every conceivable multimedia feature, but rather as a simple way to perform common functions of media players. Some developers may require more control over a backend than Phonon can provide.
Phonon will interface with various backends with what developers call "engines;" each engine works with one specific backend. Each backend will let Phonon control basic functions like play, pause, and seek. Phonon will also support higher level functions such as how tracks fade into each other.
Phonon will be able to switch multimedia frameworks on the fly. The user will be able to switch between frameworks even while listening to music, with only a slight pause during the switch. This change will also be system wide, affecting all applications that use Phonon, so changing frameworks will be much easier.
Using Solid, Phonon will give the users greater control over accessories like headsets, speakers. and microphones. An example was given that you could have a VoIP conversation only be played through your headset, but have all other sounds come out through speakers.