My customer has mentioned that when she walks across her marble floor wearing heels, whe can hear a hollow sound under several tiles and wants to know if this is normal, or should she be concerned?

You should have a professional installer check the installation for soundness. It may have not been installed properly. The hollow sounding tiles is an indication that areas under the tile are not in contact with the setting bed. These hollow areas have the potential to cause cracking.

TIP If you intend to grind this floor you may crack several tiles. I would be prepared to replace these tiles or try honing or grinding using a light machine.