When inside a armored vehicle the gun fire dosent sound the same as outside.
Its more muffled. Secondly, while not all armored vehicles ejected their coax (7,62 machine gun) casings outside the vehicle they dont fall the the ground inside ether.

The gun sounds for gunners sounds like they are shooting it in the open and for the coax it sounds like the casings, you dont really hear, are falling to the "Earth" ground and are not collected in a box inside the vehicle