We had a flood in February, due to a broken water pipe in the kitchen that seeped water into the living room area, and ruined our hardwood floor. After long discussions with the insurance company the job was given to replace the complete floor. It was replaced with Mannington Jamestown engineered hardwood. This was completed about 3 weeks ago.
Since this has been done we have noticed there seem to be "squeaks" in some areas of the floor. As well there seem to be hollow sounding spots.
Should we be concerned?

Maybe. There definitely shouldn't be any hollow spots under the floor. How is it installed? How long in between the flood/removal of the previous floor and installation of the new? Were moisture tests conducted before the new floor was installed? What type of subfloor is it?

The floor is a concrete base. They did a cork underlay which was glued to the concrete and then it was glued onto the hardwood.In February, moisture tests were run, holes were drilled in the walls and baseboard and fans were blowing hot air into the wall for 3 days. The original hardwood floor was not removed until early June and then the new cork and floor were installed over the next three days. I forgot to mention previously, that this is a condominium and the builders did not make a smooth even concrete surface when the original cement floor was placed.The wood was climatized in the warehouse for 3-4 days before it was brought to our home to be installed.
Hope this answers the questions that you asked.

I think we found the culprit.

The floor should have been removed at LEAST a week prior to the installation of the new floor.
Also, as Austin says, concrete should have been levelled by the new contractor prior to installation, regardless of whether or not it was done originally.

thanks guys