Hollow sounds doesn't necessarily mean there is a problem with the travertine. Although it can mean the tile isn't bonded or there are voids under the tile where it sounds hollow. If you are regrouting because the grout is coming loose, then that might be an indication that the tiles are loose and not bonded well. Or if you have very small grout joints between each tile then the grout might not be able to properly lodge itself into the joint.
It is true the slab should have been properly prepared to make sure the tile would bond to it and that any cracks were mitigated. The only way to determine if you have a problem is to intrusively remove some of the various stones to see the underlying conditions and to see how the tile was installed. Through a forensic company such as CTaSC.com it might not be practical to do that depending on your situation. Inspections performed by qualified inspectors normally cost $3,000 or more to perform the inspection and write a report.