Hello @Hagen,
I am the author of the SoundCloud and Mixcloud plugins, and community-developed plugins are generally unavailable in the plugin section of Volumio, unless the authors specifically submit them for approval and inclusion.
The reason why I have not done so, is that these plugins are not guaranteed to play nicely with the current version of MPD that comes with Volumio 2.x (MPD is one of the software components responsible for playing the tracks). Most SoundCloud and Mixcloud tracks should play properly, but there are some that will not because the MPD version on your Primo is too old to be compatible (said MPD version was released in Feb 2018).
I have also made available a more recent version of MPD so that all supported tracks will play, but to update it would require following a similar procedure as installing plugins manually and is also not officially endorsed by Volumio (read: no guarantee that it won’t break things).
When Volumio 3.x is officially released, then it would be time to reconsider submitting these plugins (and a couple other I have made) for inclusion, since 3.x will ship with a recent version of MPD.
-Patrick