I just took a look at the Mato Stug webpage- it says straight out that it comes with metal engines and even has a pic. I'm sure that you inadvertently got a mis-shipped plastic gear Stug sent to Mato. Of course, the question arises as to how they could test the tank (as it says they do) without noticing the quiet gears. There is a drastic difference between even broken in steel gears and plastic gears in the sound....

I'm sure that if you let them know of the mixup they will send you a set of steel gears, then you'll have a spare plastic drivetrain!