SoundCloud is a dope platform, and it would be awesome to have it as a channel on Roku. Unfortunately, I don't think it's available as an official channel at the moment. However, there are ways to stream SoundCloud on Roku using screen mirroring or third-party apps.
It's not readily apparent to most that each channel provider creates, provides to Roku, and maintains the specific Roku-compatible version of their app that runs on the Roku system. Roku just provides the platform on which it runs. If you want to view Soundcloud on your Roku, you need to convince them to create a Roku-compatible channel app and make it available.