From the API:

soundcloud:tracks:[track_id]

Correctly formatted URL:

soundcloud://tracks/[track_id]

Example:

soundcloud://tracks/63085864

Similar SO thread: https://stackoverflow.com/questions/11430436/url-to-open-track-in-soundcloud-ios-app

Playing with the SoundCloud API:

https://github.com/fredr/sounds-like-a-stream