Chapter 10 Music Discs
9 - The Oppressed
Chapter 10
Buy the disc from the vending machine in the Sewers (Aqueduct 1).
This is found in the room after draining the first aqueduct.
Chapter 11 Music Discs
6 - Let The Battles Begin!
Chapter 11
Can be purchased from the vending machine located in the Train Graveyard (Maintenance Facility 2F), just before the control room and the Ghoul boss encounter.
Chapter 13 Music Discs
15 - On Our Way
Chapter 13
Can be purchased from the Item Shop near the Evergreen Park area in Sector 6.
14 - Main Theme of FFVII
Chapter 13
Purchased from the vending machine found in the Shinra Underground Test Site.