Am I sinful if I teach someone how to make a CD for Islamic hymns, and he is known for listening to singing, and then he uses this method to make CDs for songs? And how do I absolve myself of this sin, if it exists?

Means have the rulings of their ends. Therefore, teaching a man a method is permissible if it is highly probable that he will not use it for something forbidden. However, if it is highly probable that he will use it for something forbidden, then it is not permissible to teach him. And whoever teaches, and the teaching is used for something forbidden, has shared in the sin, and bears the burden of those who followed him. Purifying the self from sin is achieved through sincere repentance.