I am living in Thailand andmarried to a Thai lady on a Thai marriage visa. Should my Thai wife pass away before I do, does anyone know what happens to my visa?

All depends on whether you actually have a Visa or an Extension of Stay.

If you actually have a extension of stay (it is not a visa) it will remain valid until it expires if your wife was to pass away.

I think its obvious you wont be able to get a new visa or extension etc. 

I believe you can stay on a current extension until it expires,  even if it was cancelled immediately, immigration won't know your wife is gone unless you tell them, or isn't with you for the next extension. 

 

The only thing that would help you is if you and your wife had a child. Other than than, if she dies you have until the extension of stay expires and then go for a extension of stay for retirement if you are old enough.

8 minutes ago, Peterw42 said:

I believe you can stay on a current extension until it expires,  even if it was cancelled immediately, immigration won't know your wife is gone unless you tell them, or isn't with you for the next extension.

It is in the rules that the extension will remain valid until it expires. If it was a non-o multiple entry visa it would also remain valid.

From the STM2 form for conditions of stay.