What should I do to repent from the mistake of not returning the Tajweed audio cassettes that I borrowed from the mosque, some of which I have lost, knowing that they are no longer used or sold?

Since this type of cassette is no longer sold or used, the inquirer should buy what is currently available and serves the same purpose, such as laser discs, and place them in the mosque library. If there is no library or one to safeguard them in the mosque, then they should be placed in another mosque, while fulfilling the remaining conditions of repentance.