Well, I didn't necessarily mean 'mix tape' in the traditional sense (where someone just compiles a list of songs together). I meant cutting individual sounds from a song (or songs) and using them to create a new song (possibly combined with some original sounds as well). I don't consider that to be 'fair' use. It seems courts often don't find it to be legal 'fair use' either, based on the history recounted in http://en.wikipedia.org/wiki/Legal_issues_surrounding_music_....

But for something to be a parody of something else it needs to comment upon or mock the original. Most songs, whether they use someone else's samples or not, don't fall into that category.