Only messages from contacts, or those on a WHITELIST should produce an audible alert. In drop-down options, add "add to whitelist" and "remove from whitelist".

I generally don't want to add everyone that texts me (about my Craig's List item, for instance) to my contacts, but I would whitelist them for a week or so.

When I am done dealing with that person, I can go to the drop-down and un-whitelist them. Also, Textra should prompt to un-whitelist that person when I delete the entire thread.