AI can be used in music in many different ways, it can be used both to compose (create music) and transpose (create written music from listening to pieces). Getting computers to compose well is an incredibly hard task. Computers find it hard to merely listen to a pieces and "feel" whether it is beautiful, or nothing more than cacophony. Computers that compose often require human input to determine whether the music sounds good or not. One program that does not quite require this is a program called variations, developed by Bruce L. Jacob. This program uses genetic algorithms to compose and listen to the piece to decide whether or not it is good. B.Jacob describes how this genetic "ear" works in his Composing With Genetic Algorithms paper: "...The [ear] module is a collection of chromosomes, each of which acts as a data filter that identifies harmonic combinations as “good” or “bad.” Before composition begins, the chromosomes are evolved to reflect the musical tastes of the human operator. First, a set of randomly-generated ear chromosomes are auditioned on how well they filter material. The evaluation mechanism in this process, as in virtually all other genetic music studies, is a human judge. Musical examples are created and passed through the ear chromosomes, and the human operator assigns weights to chromosomes according to how well they agree with his or her inclinations. Chro-mosomes with high marks are more likely to reproduce and have their alleles present in the next generation. Successive generations therefore exhibit the best traits of previous generations. Once there is a satisfactory set of filters, the [music creating] process...begins..." [B.Jacob]An example of a variations created song can be downloaded here, additional samples are also for download at the site:
A program such as variations creates music autonomously, but what about dynamically? The best example of dynamic music creation is GenJam - or Genetic Jammer. This program by Al Biles listens to him play through a device called a pitch-tracker that converts the notes from the trumpet into MIDI notes. The program then will improvise using an original set of notes. Biles will then tell GenJam whether the sequence it just played was good or bad. On the next time round, then program will get rid of the tunes that sounded bad, and evolve new parts from the good sounding songs. GenJam uses a genetic algorithm based engine to develop its notes and riffs. Transposition is in many ways even harder than composition due to the nature of the human ear and brain. A "trained" human can distinguish instruments, pick out melodies from bass lines, hear the difference between the drums and a guitar. With a computer, since everything is represented in 0s and 1s, it makes life a little harder! For a computer to listen to a 4-member rock band playing, and to create 4 written pieces of music to correspond perfectly with the guitar, bass, drums and singer would be an incredible task. To show just how hard this task would be, we'll look at one instrument, and how hard it would be to transpose that instrument alone! 
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