I gave a presentation to my local meetup about integrating OmniAuth into your application using a demo application here. You can fork it and go through the steps in section 3 of the README. There are also some slides you can look at here.
There's a lot of refactoring that needs to still be done, but it was designed to be a basic intro start to a bunch of people who had never used OmniAuth before.
I also list resources that helped with putting together the presentation. I highly recommend checking them out.
Although the examples are not with the omniauth-soundcloud, it should work similarly.