Actually Phonon is mostly a project that benefits application developers. But if all of KDE utilizes Phonon it is a benefit for the enduser:
Central place for audio/video related configuration
Configuration of hardware and media framework specific settings will be available in one place and applications don't need to provide huge configuration dialogs anymore.
Volume controls
The average desktop user doesn't understand the hardware mixer. And even if (s)he does, it's often not what is needed. A mixer that can control applications and/or application categories is what we need. So your mixer controls could look like this: Master, Notifications, Music, Video, Communication.
Automatic Device Selection
Nowadays it is more and more common to use additional audio/video hardware like USB headsets or Webcams. Phonon makes it possible to act on a signal from Solid and automatically make use of the device where requested. For example, plugging the USB headset would pop up a bubble that your VoIP program switched from using the internal soundcard to the headset for the "Communication" audio data.
There will probably be more when Phonon is finished and utilized throughout KDE. It will be added here when we know more.
