In my blog I have links to a computer software program written by Georg Hajdu that animates the STUDIE II score (with MIDI sound) as well as a link to a Ellejohara's realization of STUDIE II using Stockhausen's score instructions. I suppose it sounds as close to Stockhausen's version, as Karajan's version of Beethoven's Eroica sounds to Beethoven's original performance.
If I understood Brazilian composer Flo Menezes correctly, he told me that he re-created STUDIE II by digital means to prove that the data Stockhausen provided are correct. He told me that they were correct and that the piece sounded very much like it sounds in analog form. Maybe you ask him about this.
I'm quite jealous that Flo had such a good relationship with Stockhausen. Unfortunately I never had the courage to approach him. Or the bus fare ;).
The version of STUDIE II created by Georj Hajdu is playable with MIDI after you download his program. I assume that if the MIDI file were extracted from his executable then you could have lots of fun with it (changing sample sounds, tempos, transpositions, etc). I don't have experience with that kind of coding so if someone else extracts it I'd love to get a copy.
Many new versions have been made of STUDIE II since publication of the score. In fact, such re-creations used to be standard exercises in conservatory electronic-music studios. Some more recent versions have been touted as "more perfect" than Stockhausen's original, or even to have corrected mistakes. One important thing to keep in mind, though, is that Stockhausen never simply followed his calculations blindly—there were always dozens or even hundreds of last-minute tweaks, and often wholesale additions or reworkings, based on what he heard on playback. Gottfried Michael Koenig, who assisted Stockhausen in the studio for several works, said shortly after the completion of Gesang der Jünglinge: "I would not say that Stockhausen's work is so greatly inspired by theory. I find on the contrary that his main work is of a practical nature, i.e., the composition of an electronic piece takes place mainly in the studio, and consists in listening to sounds and listening again, and yet again. He has ‘hungry’ ears, and in his case nothing will be incorporated into the score that his ears have not previously ‘tasted’." (Letter to Allan Kaprow dated 15 May 1959, in Koenig, Ästhetische Praxis: Texte zur Musik, vol. 4 (Saarbrücken: PFAU-Verlag, 1999), 195. Another pertinent example is given by Stockhausen in "Electroacoustic Performance Practice" (PNM 34/1, 1996, 74–105), when he describes the problems with finding a digital replacement for the creaky old filters used for Mikrophonie I. Despite the supposed "perfection" of computer-based digital filters, the sound did not satisfy him, and in the end Stockhausen realized that what was missing was the noise and irregular response produced by the carbon slider in the original. A little dirt in the system was an essential part of the performance practice!
Yeah, when people accuse Stockhausen of being "megalomaniacal" and conceited they always seem to ignore the fact that he often threw out entire works and corrected many of them throughout the rehearsal process. There's that oft-told story that he spent months working on a fragment of a tape piece (KONTAKTE I think) and when he finally played it back it was the wrong tempo, so he spent another 3 months redoing the whole damn thing - all for something just a few minutes long. Now THAT's dedication!
I feel that it could be useful to have a discussion-forum on the music of Stockhausen. There are so many people from all over the world, young and old, learned and eager to get into contact with this musical world: musicologists, composers, musicians, music lovers; people who plan concerts - who write books or have to give lectures and so on. So there should be much stuff, many ideas that we can share. And when we have open questions, there may be people who studied just that and could give a hint or a stimulus.
A problem might be the English language, but i feel that is the only possibility that many people who are interested can participate. And we can exercise tolerance to mistakes!