SVE this has been a topic for us recently, during the build we spoke to our SS regarding our creaking floors he believed putting extra screws into the floor would stop it, unfortunately for us that's not the case and you can hear the creak downstairs as well as up so I wander if we had put insulation in would it not be heard downstairs. We also had the thickest underlay with our carpet and it does not stop the sound.
I wish we had gone with insulation between floors although am not 100% sure it would stop the noise. I also wish I didn't listen to our SS who claimed he would fix it and added the creaking floors to our PCI list, as I doubt m would attempt to fix it now.
We are now 6 weeks in our home and are still waiting on M to finish the things that were meant to be done after PCI, we sent them the list yesterday and are waiting on a response, we could not afford to wait any longer for M to finish things that should have been done months ago but my advice to anyone thinking of building with them is NEVER sign PCI unless your home is ready to live in with minor things to complete because once you sign PCI you are at their mercy...
We had between the floors insulation, they left large gaps, around 200x500mm around the down lights. Not sure it will be any effective.