Anderton has it right as a general principle. There might be a good argument for an implied license, if you are buying a content collection that allows you to stick together loops and samples to create your own music, and a remote chance you could claim fair use, but if there is a written license (and there almost always is) it will set the rules for how you can use the material without risking infringement. In some cases you may be barred from presenting a loop by itself, such as a long intro without any other music, that could be easily cut and copied from your composition, but more often you are just barred from re-selling the loops or samples as such. The real protection for the licensed content creator is that the license you get when you buy the collection does not go with copies made from your copy. That is, if someone extracts the original from your composition or buys it from you, he is not going to be licensed by the original creator, and will be infringing the original copyright if he uses it.
Technically your composition, cobbled together from licensed loops, is a derivative work, even though it only uses small pieces of one or multiple original works. You own the copyright only to your own original material, but not to the licensed work of another. One implication of that is that you usually cannot give anyone else the right to use portions of your work that contain licensed material. So for example if someone asks to license a sample of your composition that includes material you licenced from someone else, you probably will not be able to give it, without the permission of the original creator. There is also a different registration requirement and form for derivative works at the Copyright Office, that requires you to name the original author. That would seem to be pretty unwieldy for a work created using dozens of loops or samples from various sources.
If you are using loops or samples that require "attribution," i. e. the license says you must acknowledge the original author, as is common with Creative Commons and Freesound.org licenses, you will need to follow the requirements for doing so, or your license will not be valid. Again that could be cumbersome if you have a lot of other people's stuff in yours. If you are required to certify that all of the content in your composition or recording is your original work, as is common in publication contracts and online music services, you will not be able to do that. While it may be possible to modify the contract, you may find yourself being required to provide clearance letters or proof of licences. Sometimes you are asked to certify that all your content is licensed, and you will need to have copies of the licenses and an understanding of them in order to do that.
http://goodattorneysatlaw.com/royalty-free-music/ http://www.copyright.gov/circs/circ14.pdf