When putting tracks up on Soundcloud, you should strongly consider turning API access off. If you do not do this, you are granting not only Soundcloud a royalty free licence to use your recordings, but also anyone else who uses their API and/or links to your recordings (eg internet radio stations, remix services or other music apps).
Soundcloud’s terms and conditions HERE. The relevant parts are are “Representations and Warranties”, and “Linked Services” under “Grant of Licence”.
As a Soundcloud user, you have already warranted to Soundcloud that you control all rights when using the service. This means that publishing, which you may not control, is also included in this royalty free license for onward usage. User uploads are also covered by this, so if third parties are uploading your recordings, they are also passing on a free license to other sites and services beyond Soundcloud, which effectively creates an ecosystem of royalty free usage for your music.