Hi! I am so pleased you enjoy this game! It was a blast to write. I do hope to write some more material about these characters (especially a prequel) but whether or not I will be able to remains to be seen. I am especially happy you liked the sound design. I went all out with this VN and hunted down as many of the best sounds as I could, which I normally don't do. But sound is important! And happy you liked the music selection as well, I am very particular.
To unlock that one track, you need to get it on the secret ending. Yes, there is a secret ending. To get it, do the choices in this order: Don't say anything, Tell him about the mysterious figure, Don't say anything, Summon Noah Anyway, Keep Trying. You will get a different variant of Ending 3! (And yes, its name will show up in the end gallery too).