Is expiation incumbent upon one who swore an oath loudly at first, then continued softly, moving his tongue and lips, and is expiation repeated with the repetition of breaking an oath to abstain from something for a year?

As long as you are not certain that you uttered the oath, then no expiation is due from you. An oath is not binding unless it is pronounced verbally, and nothing is bound by inner thoughts. Moving the tongue and lips in a way that is unintelligible is not considered pronunciation. For an oath to be binding, the letters must be uttered and produced from their proper articulation points.