Below are the locations of the music discs in Chapters 15 & 16 of Final Fantasy VII Remake!
Chapter 15
5. Lurking in the Darkness
Purchase the disc for 50 gil from a vending machine in the Central Tower – 1F on 65 M Above Ground Level in the Collapsed Sector 7 Plate. You can access this area during The Crumbling Building objective in The Path to Greater Heights main scenario.
Chapter 16
31. Scarlet’s Theme
Interact with the Jukebox next to the coffee shop in the Combat Simulator Lounge in Shinra Electric Power Company to find the disc.