Legalities of using audio clips in music?

[S_N]

What is the name of your state (only U.S. law)? Oregon

Many musicians (particularly in computer-oriented music, such as the industrial genre) seem to be using a mass number of audio clips from copyrighted material (copyrighted movies in particular). What exactly decides whether this usage is copyright infringement; what decides that this usage is legal (and commercially distributable as the musician's now-copyrighted work) vs illegal use? What set of rules/self-checks could such a musician use to keep from being sued?

 

[The Occultist]

My guess is those musicians you refer to paid money (or perhaps not, that's up to the owner of the work) in order to secure a license to use the work. Not getting such a license opens them up to a copyright infringement lawsuit.