There are a number of people commenting about how Beethoven could hear music in his head, being deaf, and thus compose entire symphonies.

But at that time the musical instruments of the day were incredibly "standardized".

Basically it was all orchestral instruments such as violins, percuasion, and all the usual fair -- and by the time Beethoven went deaf he was intimately familiar with these such that he could IMAGINE it all in his head.

Even if you went deaf tomorrow, in 20 years time you would still be able to imagine a violin -- it is a very distinctive instrument.

So I think this feat -- while wonderful-- is pretty explainable.