Verified answer

History is the symphony of the heard and the unheard.

I think this is the meaning but it doesn't make sense. If you had some context maybe it would mean something. For instance, maybe the author is referring to people who's works were very popular at the time and the works of people which were unpopular and that both of them make up the "symphony of history." Poetry is very popular in Polish so context is essential to understand. Also, this is an incomplete sentence, the word "ech" does not belong and that's where the lapse is in the meaning.