Your mouth is wearing a mask so people won't realize you're the one being so boisterous. You want to be loud and rowdy, but you don't want anyone to know it's you. If you cover your mouth with this mask, people won't know you're the one being so boisterous.
You're being anonymously boisterous outside when suddenly someone shoves soap (そ) in your mouth! "I don't know who you are, but you're much too boisterous! Wash your mouth out with that soap!" they yell.