Two things remain for this:

1. audio playback (the preview function) does not work: WARNING: bool Phonon::FactoryPrivate::createBackend() phonon backend plugin could not be loaded
   I'm running the application under plasma, installed into a user environment, What might be problematic is the mix of release-18.09 and unstable packages. Any ideas on this, @ttuegel ?

2. After installation, kbuildsycoca5 needs to be run for plugin discovery. Do I need to include something special for that?

I also welcome feedback on whether dependencies belong into buildInputs or propagatedBuildInputs

just noticed an issue with the translation files location, changing status to WIP

@Mic92 What was your idea about translation filesfor KDE applications again? Right now they're located in /nix/store/28qq14yn0ywkxs3yyrns5qkfzy6hawm9-soundkonverter/share/locale/de/LC_MESSAGES/soundkonverter.mo, you mentioned moving them to $out/bin/ instead, right?

@schmittlauch I think the problem was that the binary is in $bin (which is not the same as $out/bin, but a different output of the package itself) while the locales are installed into $out. So maybe try to move both into the same output.

As you said offline to me, the translation file actually landed in the nix profile,
so the problem here is not about wrong outputs but soundkonverter not building the correct path, when looking for translation files.

I'm afraid that the localization issues are just yet another instance of #37741. So let's ignore them for now and merge the package with English translations only.

as this is not WIP anymore: ping @ttuegel @Mic92