If "unheard compression waves" counts as an instance in which something is "sound and not sound", then gay marriage can equally be "right and wrong" if, e.g., it is prohibited by the Bible but isn't any obstacle to maximizing net preference satisfaction.

No doubt you want to protest: No, but there really is a single absolute truth about what's right and what's wrong, whereas there really isn't a single absolute truth about what counts as "sound". Maybe so, but that's a highly disputable matter and you might do well to offer some actual arguments.