SoundCloud plugin available for testing. You can find it here:
Follow the instructions there to install it (also go through the rest of the readme for a better understanding of what to expect from this plugin). For Volumio 2.x, it is also recommended that you update your MPD. Otherwise, you may encounter seeking issues for some tracks.
EDIT: note that this plugin does not support user login, for reasons stated in the readme.
You can raise issues here as you find them, and feedback is of course welcome.