This piece of music is from the Into The Abyss Level after you start turning on the generators & the proxy (Kate) spawns. All credits go to Mark J Hadley. DISCLAIMER: No Copyright Infringement Intended, all Game Content and Music Content is property of their rightful owners, I own Nothing in this Video. [Slender Developed by Mark J. Hadley].
If you're wondering what the songs are that play on the radio in the game, you're in luck. I've been listening to the radio in the game & I've come across 3 songs that play. 1st is "When the levve breaks" by Kansas Joe McCoy. 2nd is "Wildhood Flower" by The Carter Family. Lastly theres "High Water Everywhere Pt 1" by Charley Patton. You can find these on YouTube aswell if you're after listening to them. Hope this helps ;-)