Can I use a song or other copyrighted work in my film if I change ... percent of it?
Internet Filmmakers' FAQ
For a long time a copyright myth has circulated that there is a magic percentage of a copyright work, that if the original work is changed by that amount, it becomes ok to use without permission.
This is completely and utterly false.
There is no rule of thumb or magic number which separates non-infringing use from infringing use. This interpretation is entirely up the courts, and will depend largely on the nature of the copyright work and how it is used.
In nearly all cases, if you are planning to use copyrighted material in your film, you will either need to get permission from the copyright holder, or substantially change it to the point where it is not recognisable from the original source (which kind of defeats the purpose of using it anyway).
International copyright law does allow for a concept of "fair use", which allows copyrighted material to be used for certain purposes without permission of the copyright-holder. Fair use is a qualitative measure so it again comes down the to the way in which the material is used, how much is used, and what the nature of the copyright material itself is. Generally, fair use covers areas such as research, criticism, educational, and news reporting areas (i.e. mainly non-profit stuff). It certainly doesn't extend protection to fictional films, and use in documentaries is a very fine line (since most documentaries don't afford the same "in the public interest" protection that news-gathering organisations receive).
The points in this answer, also relate to a variation of the percentage myth: "If I use less than ... seconds, then it's ok." Again, this is untrue.
The upshot is, if you plan to use copyrighted material in your film, either get permission, or change it substantially so that it's no longer recognisable.