phonon-vlc (this is the only maintained and supported Phonon backend for KDE apps that use Phonon for their multimedia needs; the alternative phonon-gstreamer has been unmaintained since 2013 and should not be shipped by default, or at all, ideally)
We could also consider removing the gstreamer backend entirely or deprecating the option.
services.xserver.desktopManager.plasma5.phononBackend defaults to gstreamer. We should swap this to VLC.
From KDE's packaging recommendations:
We could also consider removing the gstreamer backend entirely or deprecating the option.