Chapter 3 Music Discs
There's no disc in the first two chapters of the game.
Chapter 3 and Chapter 4
Purchase from the store near the Sector 7 Undercity Station. You can also buy it at the start of Chapter 4 on your way to taking the train.
Chapter 3
Simply interact with the Jukebox found inside Seventh Heaven.
Chapter 3
To the right of the Seventh Heaven in the Sector 7 Slums there's an alleyway. Head down it, and you'll see an NPC outside a building. You shouldn't miss it, as the music will change as you approach.
Chapter 3
Buy the disc from the Item Shop in the Sector 7 Slums.
Chapter 5 Music Discs
Chapter 5
Found in one of the auxiliary rooms found in corridors leading away Corkscrew Tunnel Track E. You'll find a vending machine where you can buy the disc.
Chapter 6 Music Discs
Chapter 6
This one is found during the main story when you are tasked with switching off the Sun Lamps in Sector 4's Plate Upper Level.
Before taking the last elevator to reach the Reactor, take the smaller elevator to your right. Climb the ladder (on the wall with the giant fans), where you'll come to a room with a vending machine.
Chapter 7 Music Discs
Chapter 7
Buy it from the vending machine near the end of Chapter 7, at the entrance to Mako Reactor 5 (Gate Office). This is directly after the scene where Cloud, Barret and Tifa have to activate levers at the same time.
