Is it obligatory to repeat past prayers that might have been invalid due to ignorance of the rulings on reciting Al-Fatihah aloud in audible prayers, knowing that the error was not intentional and no specific prayer can be recalled?

You are not required to repeat the prayers if it was merely a doubt after completing them, for doubt about an act of worship after its completion does not affect its validity. However, if you became certain of having violated a duty in the prayer, the majority of scholars obligate its repetition, and this is the more cautious approach. Ibn Taymiyyah, on the other hand, is of the opinion that repetition is not obligatory for someone who was ignorant of the ruling.