According to the SM, there is no document mentioning wooden floors or any insulation installed.
Is there a by-law? If not, the previous owner has done nothing wrong.
apparently my kids’ piano and saxophone playing can be clearly and loudly heard by neighbours.
Hmmm. You are joking, aren’t you? If you want your kids to be able to practise at home, you should be looking at soundproofing the apartment, not just the floor. Or at least one room. Carpets on the floor and special sound-baffling wall and ceiling panels would be the go.