What should I do with the tapes that I borrowed years ago and have not returned to their owners, knowing that some of them are difficult to reach, and that they will most likely not be saddened if they find out about this?

You must return whatever you have borrowed from others, whether it is little or much, as an act of fulfilling a trust. If you are unable to reach the owners or identify them, then give what they are owed in charity. If they later appear, give them the choice between accepting the reward of the charity or being compensated. This is the fatwa given by imams such as Ahmad, Shaykh al-Islam, and Ibn Mas'ud (may Allah be pleased with him).