Dear friend,sneezing is all about ejecting air from lungs forcefully,in big amount but in small time.and this air comes out through larynx and tracheal pathway.you should know our voice producing part vocal cord lies in the larynx.so what I believe forceful passage of air did cause some tear or displacement of vocal cord making it unable for proper sound or voice production.due to old age of your father the laryngeal muscles have become weak.that s why your father has lost proper voice production abilty.you shoukd take him to any ENT specialist for check-up,where a direct observation of larynx will easily clinch the diagnosis.