Phonon can be found everywhere! Quite a mess, really. Here's where you can find what:
Phonon itself (aka libphonon, "phonon proper", or [confusingly] just "phonon") is the library that lets applications use backends. It can be found at git://anongit.kde.org/phonon
The different backends are all stored on KDE git as well. Pick your poison: http://projects.kde.org/projects/kdesupport/phonon
In git://anongit.kde.org/kde-runtime, you can find the KCM, phononserver KDED module, libkaudiodevicelist, and the KDE platform plugin.
Phonon.kde.org can be found at https://projects.kde.org/projects/websites/phonon-kde-org