Best answer by Mathis
Thanks for posting on our help forum. The primary use of the forum is to provide help with technical issues and questions around SoundCloud itself, but I'm sure we have some users who have experience when it comes to audio editing and might be able to help with your question as well :-)
Adding music to your vocals is often done in a so called DAW (Digital Audio Workstation), but any audio editor on your computer that supports the editing of multiple tracks should do for now - one such software is Audacity, and it is freeware as well, so it might be a good starting point for you. Please note that it is not software that SoundCloud develops though, so if you have questions about that, the good people over at Audacity will be able to help with this.
Mixing vocals is not something you learn over night - there is a lot to it (EQ, Compression, Limiting, etc. etc.)! I've done a quick Google search for you to see if I can find a good entry level article and I think this one might help to get you into the subject:
As you can see, there's lots and lots and lots of info and detail when it comes to mixing music and vocals :-)
Hope this helps you to kick it off!