While the guns on tanks are certainly loud, remember the blast and projectile are going through the barrel on the exterior of the tank. The shell itself is inside the cannon's chamber when it fires as well, also hampering interior noise. The barrel blast is definitely heard inside but a large amount of the noise is the breech absorbing the recoil. So for example the KV-2's crew weren't subjected to the full force of the 152mm shell going off inside, there are multiple factors inhibiting the sound.