Location: The Abandoned Chapel, Act III. The Problem: The organ has four pipes missing. You have to find pipes hidden around the chapel, but the notes must be played in a specific sequence. The Solution:
Pro tip for True Ending: After the credits roll, wait 10 minutes on the title screen. A secret cutscene plays showing the main character mailing an apology letter to a relative. This confirms the "real world" interpretation.
Because the game likely does not exist under this specific name, traditional game guides (IGN, GameFAQs, Prima Games) do not cover this title.
However, if the user is looking for the "Regret" aspect in gaming, resources are available for: