I want to start an aio website that will consist of mp3, music videos and 18+ videos which will be an easy one click download. Do we need to get a license or is it enough if we mention the source of the content?
If you mention what?
Yes. Well... it's more complicated. Some artists have restrictions and 'royalty free' does not necessarily 100% free. I have a list somewhere that explains usage rights. Most sites have this in their terms with restrictions. Please double-check where you are getting the source files from.
If the content is someone else's you will need a license to share something you don't own UNLESS you get Creative Commons material. If you're creating the content you do not need a license to publish it however you will want documentation for others who want to publish your original content.