Can I Re-enter Thailand on a Tourist Visa After Leaving on a Volunteer Visa?

Yes, according my crystal ball to see the future and the tea leaves to see you history of living inside Thailand..

But to be honest, with this information nobody can give you any advice, even worse, we can no tell if the employee of the Thai Consulate is willing to sell you a new visa and we can not tell if the Immigration Offices at the check point viewing all your history in entry, leaving, usage of visa, name, dob, gender, digital pictures, fingerprints and more will say welcome in Thailand or Sorry, but it seems to me you are not a tourist.

If the visa is still valid and is multiple entry, then so long as you carry the old passport complete with visa it should be fine.