Phonon needs backends to provide its functionality. Here is a list of the current backend projects:
Phonon-Xine
Xine is one of the best known play-what-you-throw-at-it libraries on Linux. KDE 4.0 will ship with a backend using xinelib for all the heavy lifting.
Phonon-NMM
The NMM group at the Computer Graphics Lab at Saarland University and Motama have started a backend for Phonon. This backend is currently unmaintained.
Phonon-GStreamer
GStreamer is a very flexible media framework (compared to xine, at least) and will be supported from Qt 4.4 on (and integrated in KDE from KDE 4.1 on).
Phonon-DS9
A Backend for Windows that uses DirectShow 9.
Phonon-QT7
A Backend for MacOS X that uses QuickTime 7.
"Third party"
VLC and MPlayer
A project outside of the KDE repository to develop Backends using VLC or MPlayer: Homepage
