hardboard would do little or nothing in the way of soundproofing. i believe there are products available which will go some way to help, but the best things is carpet with a nice thick underlay.
my son left his previous flat because of the noise from above due to laminate floors, i do wish people would realise what a nuisance it is. as you say, he could hear absolutely everything - phone conversations, what programme they were watching, but the worst was just the walking about in shoes.
if you possible can, have a good chat with them and try to pursuade them to get carpets, invite them in to hear what you can hear, to try and make them realise what they subject you to every day. best of luck.