I am going to try to learn Rachmaninoff's C minor for the piano, most likely simplified. I always love the song but what is the correct way to play it at 7:15 of the video. I see a lot of videos that plays that part softy or loud and hard expressing themselves. I know it doesn't really matter but what was the original way of playing it.
You can find a score to this movement here at IMSLP. On page 32 of the document, you'll see that this section is marked ff:
With that said, this is a reprint of a score from Muzyka, and it seems that the first publication was from Gutheil. To know with 100% certainty, you'd want to check the original Gutheil score (and the autograph score from Rachmaninoff, if that exists).