How do know if bands like Slip Knot is singing when they wear masks and their lips are sewn shut ?

There is another video that MTv constantly plays ,that a group of people having bandannas concealing their faces,and wearing theatrical masks and have duct tape over their mouths...





How do know they aren't pulling a ';Milli-Vanilli'; in concerts,when there's proof that the bands are doing their work .How do know if bands like Slip Knot is singing when they wear masks and their lips are sewn shut ?
i just went to a slipknot concert on valentines day, and their lips aren't sewn shut during a concert, watch the music video dead memories by slipknot and you can get an up clost of the lead singers mask, there's a large opening for his mouth. and the band HU they don'y wear their full masks during concertsHow do know if bands like Slip Knot is singing when they wear masks and their lips are sewn shut ?
Many artist- most artist use background vocals like the one ashley simpson got busted for. I seen puffy do it, eminem do it where most people wouldn't notice. Both of them were at the grammys, but milli-vinilli never even sang music, it was another artist who sung those songs but used milli-vanillas face for exposure
That was Hollywood Undead on Mtv and they can because its pre recorded in music vids


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I've got news for you they wouldn't be able to sing if their mouths were not covered either. At least with Rob and Fab somebody was singing. It may have been recorded but at least whoever it was could sing.
Well it depends how sucky they are but they can also record a fake buy idk