The aim of this article is to help you better understand the copyright principles, such as fair use, public domain and creative commons licensing, that you need to be aware of when including music in your YouTube videos. At the end of the page, there are also a few links regarding where to find music...
This is a third tool like Garageband that creates layered tracks. The feature that makes this tool unique is "the revolutionary Session View: a unique sketchpad for improvising, playing and performing with musical ideas, without the constraints of the timeline. Freely and independently start and stop any number of audio or MIDI loops - everything stays in sync."
In Brass/Woodwinds Methods Lab, Dr. Mantie recommended this as a good backtrack creator to use in the classroom. It's not too expensive and would be a useful tool.