Thank you, Enrique (Gracias, si eres español, como yo)
for your reply and effort. Unfortunately, it was his piano sonata, lento movement, not the song above. And yes, I was also surprised because it took me a long time to find the information you've just given to us haha Thank you very much.
I believe pretty certainly, i think, it is on public domain, as there are so many Rachmaninoff's works at IMSLP and as it is not a "mature-period" work (then, not so requested,famous and then without true economical worth?).
Anyway, its score exists somewhere, like all musics, specially from composers of his relevance, and if someone has the way to get it, either me or people who has messaged me for if I had already got it, would be truely grateful.
Thanks in advanced,
Mussels with nutella