We know that it is major sin to look at pictures of beautiful top model women on the internet and beautiful songs. No matter how beautiful they are it is major sin. But someone might skip this by imagining the song or the beautiful women in his mind. Playing a song or a picture in your imagination produces almost the same pleasure and sometimes even more than the real world. Is this a major sin???
Imagining doing a sin is exactly as doing it in the real world as the scholars stated. Thus, what you are doing is Haraam (forbidden), and you should stop it since it is the same as watching those women models and hearing those songs (which may include music and obscene words).
You actually have set up the argument on yourself; you wrote that you get the same pleasure if not more:
...produces almost the same pleasure and sometimes even more than the real world
and this is what we avoid by not doing sins besides the real harm that may follow.