Searching and launching the program phonon works fine, but favoriting it and launching it from the favorites menu results in a Plasma error: Unable to run the command specified. The file or folder kcm_phonon.desktop does not exist.
I can't reproduce this with Plasma 5.7. The generated favorite (kcm_phonon.desktop) launches fine here, and I see no technicall reason why it wouldn't. I think this might be an environment/setup problem on the distro. I'll ask some other people to test to make sure.
I cannot reproduce this either. Can you please check if you have the following commit [1]. Is this just the Phonon KCM or any KCM (eg. Colors)? [1] https://quickgit.kde.org/?p=plasma-desktop.git&a=commit&h=6ff17e25466942471f358806244ff894b7a3d8b2
How do I check if I have that commit? I think it's just the Phonon KCM. Also making a desktop shortcut from kicker (kcm_phonon) and launching it results in the same Plasma error: "Unable to run the command specified. The file or folder /usr/share/plasma/plasmoids/org.kde.plasma.icon/contents/ui/kcm_phonon.deskop does not exist."