There really isn’t any way to make direct restitution. You have deleted them, and that is about all you can do.
I am just offering this as advice from a layman, nothing more — and I wouldn’t want anyone ever to develop a scruple over such things — but one way of making some kind of restitution might be to make a small donation to the music or performing arts department of your local college, or even leave a nice juicy tip to the next busker you see (street or subway performers who usually have a guitar case or similar thing open for this purpose). That, too, is compensating a musical artist or student for their hard work. You got rid of the recordings — it is not as though you will have them to enjoy for free the rest of your life.
Just throwing that out there. I would be at peace about the whole matter.
I’m proud of you for that, I found out/realized that a few years ago and it’s still a deep struggle for me to come to terms with. As far as paying back goes, I think you’re on the right track; deleting them was a firm action against it. I like the other advice as well, supporting other musicians as a sort of penance.
Unless a priest imposed it as reparation in the confessional, something like that would be “going over and above”, doing something positively good, “doing more than you have to”. The big word is “supererogatory”.
And sometimes, even from a secular, humanistic point of view, it just feels good, just feels right, to “do more than you have to”. As I mentioned elsewhere on the forums recently, even if I were an atheist, I would still follow probably 95% of Christian morality and ethics, simply because they’re good in themselves, and so much better than anything else the world has to offer. Gandhi would have told you probably pretty much the same thing. He had some pretty tart words about Christians.
If you truly didn’t realize it was “a sin” when you were doing it, then by definition it was not a serious (mortal) sin. You’re off the hook, and I absolve you from your sin (grin). The fact that you are now regretful and want to make restitution shows that you are in the proper frame of mind. Keep up the good work