However, what if no one can know whether or not an argument is sound because no one can evaluate its claim as valid and its premises are true?... why is it that if an argument cannot be known as sound it could still be considered sound?
So, central to your question is the notion of knowledge. All the rest circulates about the notion of knowing, and before any philosopher would wade into explicating on this matter, it would behoove said philosopher to address what the meaning of known is. It also might help to drag out the difference between appearance and actuality.
So, the problem of radical skepticism essentially argues that knowledge isn't possible, and for a radical skeptic, then sure, no argument has any sort of privileged role in sorting out what is actually known from what is merely believed. Problem solved. Words are mostly useless, all claims are equal. There is no actuality, but only appearance. Except this is an absurd position reserved for Greek and Roman skeptics and teenagers on a crash course for existential crisis.
What modern philosophy of logic has to offer is the notion of defeasible reasoning (SEP), which is the idea that claims can be treated as truths until defeaters come along and force the thinker to reevaluate what is and isn't true. Now, in the olden days, philosophers craved certainty and wanted truth to be objective, like reality itself. If a tree is a public experience, than truth should be too. The problem is unlike a tree, which is subject to the rigorous of scientific methods and empiricism, truth is a concept and is at best an abstract object (SEP). Truth doesn't grow on trees.
So how is truth and knowledge to be had? It seems to boil down to the notion of justification or judgement which itself is a bit murky. What does it mean to justify a belief? Dismissing Gettier's objection as academic, the general notion that we undertake some specific process to ensure that belief is true and thus a belief can be claimed as knowledge is highly domain specific. Stephen Toulmin in his Uses of Argument provides us a very tidy semantics for discussing informal argumentation. Justification can be seen as vetting belief through argumentation by providing backed warrants and examining rebuttals and exploring a topic until we sift the wheat from the chaff. In fact, in informal argument, unlike it's more formal relative, we don't even use the terms valid and sound, but instead talk of strength and cogency giving the words themselves a more probabilistic flavor. And that's the point to be made.
If you are apprising how logical an argument is, you are trying to prove truths are objective, you are just trying to shoving them past reasonable doubt, a phrase you might here uttered in the more real-world context of law. That's because justification or judgement in real world thinking, not the stuff of obnoxious pedantry, relies heavily on philosophical intuition (SEP), and why many people believe an argument when even the argument made is obviously philosophical bullshit. There are pervasive fallacies and cognitive biases, deception, confabulation, and hallucinations to contend with, and to be sure that something is true without the possible of future revision of belief is a bad idea in the real world. In fact, this notion that human belief is fallible is central to the doctrine of fallibilism.
What's important to know is that argumentation as persuasion relies on non-rational factors including intuition, emotional appeal, and even the character of the person making the argument, the familiar ethos, pathos, and logos we know and love. And even when it comes to the language-games surrounding logos, certainty is hampered by honest error and deliberate action. The idea that you can appeal to aprioriticity for certainty in the Euclidean axiomatic method is an important facet of reasoning, but it's neither typical nor exhaustive of reason in everyday life. Most people rely on surface clues and shallow justification to arrive at truth, largely on an economics of attention. And that is why many people see soundness where further inspection reveals doubt. Arguments are just words that guide us to other words mired in still other words and our feelings and experiences.