Basically, Nietzsche speculates on how Beethoven would react to how his music was being performed. Nietzsche puts the following words into Beethoven's mouth:
Das ist weder Ich noch Nicht-Ich, sondern etwas Drittes …Ross provides Gary Handwerk's translation for the new Stanford University Press complete edition, currently a work in progress:
That is neither I nor not-I, but some third thing …I would not quibble with this, and I rather like both the original turn of phrase and the way in which Handwerk translated it. However, had I the ability to talk back to Nietzsche ("a consummation devoutly to be wished" by many, I am sure), I would probably have replied that, had Beethoven's hearing been restored by some miracle of medicine, he may well have said the same thing in 1824 after the first performance of his Opus 125 ("Choral") symphony in D minor.
The only time a performance of music is truly the "Ich" of the composer is when the composer is the performer. This is as true of Beethoven as it is of Johann Sebastian Bach or Elliott Carter. Once the act of music has been abstracted by the composer into marks on paper, the composer no longer has absolute authority over how those marks are interpreted. By way of a reductio ad absurdum, I recently wrote an article on my Examiner.com site with a visual illustration of how composer Danny Clay developed a project for elementary school students around the marks on paper for the Opus 133 "Große Fuge" in G minor, which Beethoven would probably have declared to be "Nicht-Ich" without hesitation!
The bottom line is that this is simply another illustration of the proposition that the music is in the making, whether the composer is Beethoven or John Cage; and, when the composer is not actively involved in that making process, (s)he no longer has the final say in the matter.