As far as the tile goes, it's been a while since that was an issue, but if I recall correctly, there were issues underneath the tile...wish I could be more specific.

If your subfloor is constructed out of 1x's, plywood or OSB you can screw it down to the joist underneath if you have access to the subfloor.

Although with the Caveat that in Texas with their non owner occ tax's and soil conditions one needs to be pretty cautious there..

It will either suck all the moisture out from the soil underneath the foundation or damage the foundation by growing roots under it. 

This is the first opinion I've heard that our investment is a terrible one and I'd love to get underneath why that is. 

So the reason I mention all of this; the EPA has come forward and announced that the soil in these communities are contaminated with high levels of Lead, as much as 7 times higher then then normal level.

Here are some reasons why it may have cracked......depending on your area the most important aspect of the foundation is actually the soil it sits on.

Put down roofing felt paper underneath as a cheap pad it helps with sound.

Removing contaminated soil to government specs is beyond expensive.

Soil, pests, tree roots, water, etc.