If you want to write a new effect that should be usable from you Phonon
application, I encourage you to use LADSPA. This plugin API was designed
especially to make the effect usable for as many audio
libraries/frameworks/applications as possible. The result is that your plugin
can then be used by more than only one backend.

It is not coincidence, that the effects API of Phonon is modeled after LADSPA,
since those are the cross-framework plugins...