Okay this is what I know. All this stuff is really complicated and very vague and subjective. There are also tons of myths about copyright floating around.

If you make any kind of collage, including a music collage, that is, you use one work of art to make another, this is called a derivative work. Legally only the person holding the copyright on the original work has the right to make a derivative work or alternatively you have to get their permission. This means that the simple act of creating a derivative work from works for which you don't have the copyright or have permission is already an infringement. Should I repeat that? Okay. This means that the simple act of creating a derivative work from works for which you don't have the copyright or have permission is already an infringement.

Lots of mixers thought that just using a teeny bit of a song was okay and ended up getting their ass sued. So today they ask permission first before they use that groove from the Undisputed Truth. If it is lesser known stuff between "colleagues it probably isn't a big deal but you better ask permission before you publish it.

There is something called "de minimis" which means that small bits of works can be used, but this is not well defined. A good rule of thumb is that if no one can recognise it, then it is okay.

Myths abound, like you can use 5% or 10% but that is all nonsense. If that 5% is an essential component of the new work, it is a derivative and you can kiss mama goodbye.