"Un-sounder" is not a commonly used word in the English language, but if we were to look for antonyms for it, we would have to consider its components. "Un" is a prefix that means "not," while "sounder" means "firm, strong or reliable." Therefore, antonyms for "un-sounder" would be words that have the opposite meaning of "not firm, strong or reliable." Examples of antonyms could be "firm," "solid," "stable," "reliable" or "dependable." These words convey the opposite meaning of "un-sounder" and can be used in a variety of contexts to describe something that is stable, dependable and trustworthy.