Mix tapes are a delicate science. Nick Hornby declared as much in his 1995 novel "High Fidelity," wherein he pontificated on the proper placement of songs, the flow of one song into another, and his intense concerns as to what kind of messages a properly thought-out playlist would communicate to the intended recipient. To quote:"To me, making a tape is like writing a letter. There's a lot of erasing and rethinking and starting again. A good compilation tape, like breaking up, is hard to do."As Sarah Vowell once said on "This American Life," a homemade tape is a work of friendship.