Is it a sin if you don't pay back what you've stolen? Like I used to copy movies and, well, that's stealing, but I don't feel like paying back what I copied. Isn't that why Jesus dies for our sins?
Repentance from sin means making a complete turn around in your attitude and behavior toward sin. Paul said he "declared first to those in Damascus and in Jerusalem, and throughout all the region of Judea, and then to the Gentiles, that they should repent, turn to God, and do works befitting repentance" (Acts 26:20).
A part of the change is a desire to fix problems that you may have caused when sinning. "For godly sorrow produces repentance leading to salvation, not to be regretted; but the sorrow of the world produces death. For observe this very thing, that you sorrowed in a godly manner: What diligence it produced in you, what clearing of yourselves, what indignation, what fear, what vehement desire, what zeal, what vindication! In all things you proved yourselves to be clear in this matter" (II Corinthians 7:10-11). Zacchaeus demonstrated this when he turned to Jesus. "Look, Lord, I give half of my goods to the poor; and if I have taken anything from anyone by false accusation, I restore fourfold" (Luke 19:8). Zacchaeus declared that if he wrongfully took money from someone, he would return to that person four times what he stole. The Jewish Law only required that a thief return 20% more, but we can see Zacchaeus' zeal in wanting to change because he was willing to do more than the law required. Of course, there would be many whom Zacchaeus would not remember or not able to restore. For these, Zacchaeus declared he would take half of what he had and give it to the poor. Perhaps those he stole from would benefit, but in this manner he removed the ill-gotten profits from his own household.
If you copied things you shouldn't have, the best thing to do is get rid of the copies. If there are things that you really like, go out and buy a legitimate copy of it so that the ones producing the material are able to benefit from their labors. Spread the word why taking things without permission is wrong and thus demonstrate by your attitude and behavior that you want to do what is right before God.