"Roman blacksmiths from their shackles cried,"
had me stumped.
I attach both a figurative and a literal meaning to the word "shackles". The literal meaning is that physical iron shackles dating from Roman times held the voices of the Latin-speaking blacksmiths that made them. The figurative meaning is that their voices--the ghostly echo of their own selves--were bound up (shackled, as it were) for two thousand years.
Does it mean that your "dream invention" unlocked sound from metal artifacts produced by (enslaved?) Roman blacksmiths and from Franklin's harmonica, but from nothing else ever?
As for the last line, it's not that nothing more was ever heard, but that it was never anything particularly useful or enlightening. In my dream, even Franklin's fragment of a question to a glazier turned into a farce: the chief effect was to spark a flurry of plays and TV shows about Franklin, who inevitably, and in a queerly distorted voice, would end up asking a partial question of the man he'd commissioned to make the Glass Harmonica. The economy ended up geared towards producing useless little snippets of sound from the past.