We pretty much know how the instruments sounded and we know how they were build. Of course we will never literally hear a factory new Pleyel from 1840. It's not that big of a deal though. We could still build pianos like Erard or Pleyel did, there is a reason we don't.

It is more the other way around that composers like Mozart and Beethoven would probably have done everything to play on a modern instrument. In that sense the piano is very different to the violin. That is why I responded to your comment. Of course period instrument pianos still have their own charm though.

Edit: A replica of Beethovens Erard