The condo board (HOA in the USA) dictates your noise requirements. If you are using engineered hardwood you can look at a floating floor installation (to reduce costs...it's one of the reasons engineered hardwood was invented).

In most of the US, I've found the requirement for noise reduction is 6mm cork (1/4"). In some of the higher priced condos the requirement is more and therefore 12mm (1/2") is required. This gets into all sorts of issues with floor heights (like through doorways, in the foyer, in the kitchen, etc).

Cork is state of the art. Believe it or not, it is pound for pound dollar for dollar the most efficient noise reducer we have. It is the lowest VOC. It is the most economical and ecological of all options in the building world. Yes it is 'old school' but it is also the most viable for 95% of the population.

If you want glue down hardwood in your condo, the cork will have to be glued down first. Then you have to wait for the glue to cure (24-72 hours depending on the product) and then the hardwood has to be glued down to the cork. We call this a 'double stick'. The cost of the glue, for each application = $1/sf. Just to glue down the entire thing you are adding $2/sf onto the job. Labour doubles because the glue is two layers (s/he has to glue down a floor twice).

A floating engineered hardwood doesn't need any of that. The cork sheets are loose laid (no glue...just edge butted to each other) and the hardwood is floated right over top - same day. Cost of materials goes down (no glue = $2/sf savings) and the cost of labour goes down (only lays the floor once). Labour cost to install floating cork underlay = $0.50/sf. That's it.

If you want state of the art, you will have to pay the price for it. You can look at mass-loaded vinyl. But vinyl and wood do NOT play nicely together. You can get some odd odours coming from the glues needed or the two products themselves will have a chemical reaction to each other and cause an odour all by themselves.

You can look at the super thick QuietWalk. The acoustic numbers are very impressive but you need to work with the expensive stuff to get it up to condo standards.

You can look at rubber but you are going to be spending HUGE amounts of money ($3-$4/sf) just for the low-odour product. Then the specialized glues needed to get rubber to stick to concrete AND wood = another $2-$3/sf. And you STILL risk odour coming from the rubber for decades (every time the sun hits the floor the rubber warms up and begins off-gassing faster).

The 6mm cork = $0.80/sf. The 12mm cork = $1.50/sf. That sounds expensive until you compare the TOTAL cost of install (glues, labour, etc).

And remember: the adhesives are the single biggest producer of VOCs in the building industry. As soon as you require a 'glue down' hardwood you are introducing glue into your home. To get away from the glue you simply float the floor. With a thick cork underneath the floating hardwood will feel just as solid as a glue down.

I'm a cork flooring professional btw. I've seen your situation many times. By trying to go 'State of the Art' many homeowners discover the mess they have made for themselves. It is very sad. Personally I would loose lay the cork (over the properly prepared/flattened slab) and then float the hardwood. It is SUPER easy and achieves everything you want....while saving you close to $5/sf.