Metal Gear Solid: Peace Walker

[TheDax]

Moreover, voiced audio doesn't play when receiving codec messages (like in the beginning where Kaz tells you to shoot the cocounuts and such)

Did you install the 300-something MByte data install that was offered when you started a new game? It's required by the game for most voices in-game. You can ignore the 800MByte one, it won't make the game load any faster.