Hi. Installing "hardwood" over concrete determines what you can do. My understanding is that you can #1 install plywood--then cover with hardwood (which may help with your noise level, if it's a problem) however it is costly, as you are putting on an extra layer. #2 use an engineered hardwood, which can be applied directly to the concrete. With some you lose the inability to completely refinish, and the owner can only recoat, so if longevity is an issue option 1 may be more cost effective in the long run if your owners are given the option.