Music on Soundcloud is owned by a specific artists (or group of artists) or label. To place music in the background of your videos on YouTube you should really look to license the music. To license music you would normally need to talk with the artist and then make sure they control all the copyrights and then agree a price. This process normally happens via email and it takes quite a long time.
However, any creator can now make this process simple and easy by just heading to Synchedin and purchasing a license. This will allow creators to download as much music as they want and then use them in as many videos as they want across all platforms (not just YouTube).