Of course you walk on your heels. People dont normally walk on the balls of their feet.

Do you wear slippers or other soft shoes in the house? Are there rugs or carpeting? Can you add some?

Maybe get a white noise machine and place it near an air vent in the area where they might be getting the most noise?

Really they cant expect it to be absolutely quiet. I hear my side neighbors, but not up or down. We are carpeted here.