Phonon VolumeFaderEffect not working with VLC backend

KAlarm uses Phonon's VolumeFaderEffect. This works with the GStreamer backend, 
but I can't find a way of making it work with VLC. Looking at the phonon-vlc 
code, it looks as if it should work without problem. It's coded as follows:

    mAudioObject = new Phonon::MediaObject(this);
    mAudioObject->setCurrentSource(source);
    auto output = new Phonon::AudioOutput(Phonon::NotificationCategory, this);
    mPath = Phonon::createPath(mAudioObject, output);
    output->setVolume(mVolume);
    mFader = new Phonon::VolumeFaderEffect(this);
    mFader->setVolume(mFadeVolume);
    mFader->fadeTo(vol, mFadeSeconds * 1000);
    mPath.insertEffect(mFader);

The values of mVolume and mFadeVolume are between 0 and 1, but whatever I set 
them to, the fader has no effect on VLC. I've tried calling the last line both 
when initialising, or when the Phonon state changes to PlayingState. Can 
someone please advise about what is going wrong?

-- 
David Jarvie.
KDE developer, KAlarm author.