To unlock sound gallery in the extra feature, you must locate and destroy the Green Capsules in the individual Extra Missions using a materia shot to destroy it. Each one will unlock a different sound clip.

Battlefields
Green Capsule in Mission 4: Vincent the Mage, C-12

Bosses
Green Capsule in Mission 29: Trick Arc, L-7

Events
Green Capsule in Mission 7: Gatling in the Wastes, F-9

Omega
Green Capsule in Mission 42: Two-Handed, K-6

Shinra
Green Capsule in Mission 22: Shinra Manor Prime, H-15

The System
Green Capsule in Mission 2: Widow Missileer, I-6

Tsviets
Green Capsule in Mission 39: Brotherly Love, G-3

Vincent
Green Capsule in Mission 34: Sprial, D-5

WRO
Green Capsule in Mission 17: Red and Blue, G-15