This must surely be The Ninth Symphony of Ludwig van Beethoven and Other Lost Songs, a short story by Carter Scholz, first published in Universe 7 in 1977.
As you recall, investigators use a mental transfer technique to communicate across time to great artists of the past, such as Beethoven:
The machines were windows into the past... for what they did was
transfer your consciousness into the mind of someone in the past.
With just a single researcher riding in the mind of a subject there was no problem - the subject was completely unaware of the "rider". But a problem called "crosstalk" occurred when several researchers inhabited the mind at the same moment:
"The more channels, the worse it gets. So they're especially worried
about guys like Beethoven; sometimes he has a dozen or more
researchers in his head at once. They're worried about that."
"What, that Beethoven might overhear thoughts from the future?"
Researchers were particularly interested in a date in 1823 when Beethoven began composing his Ninth symphony. When he visits this date, the protagonist, Largens, hears thousands of voices, so intense that Beethoven's mind cannot tolerate it, and he goes insane. As a result:
They had killed the Ninth Symphony. All the frustrated pianists and
composers and singers turned scholars had brought their frustration to
the works they studied; and they had brought it to Beethoven himself.
All their souls' cheapnesses summed; the faint crosstalk turned to a
shout. They had brought their weakness and despair in such pent-up
furious quantity that Beethoven had been swamped by it, and drowned.
When Largens returns to the present he finds that the past has been changed; in this new timeline Beethoven had gone mad before writing the Ninth symphony. The story ends with Largens pondering if research continues, will Beethoven's oeuvre decrease until finally he never becomes a composer at all?
