I've received multi-layered gags- a hanky or bandanna in my mouth, one tied cleave-gag style holding that in, and another one tied over my mouth detective-style. Sometimes an ace bandage was used instead. And I could still make a pretty decent amount of noise, you just couldn't understand what i was saying. A sound-proof room (putting quilts or blankets on the walls, for instance) or a masking noise, like the TV, radio, running water or nearby construction work or traffic would have to cover all the sound. Even rhythmic thumping or tapping will bring someone to check on it- Nancy Drew did it in THE TAPPING HEELS.