To be fair I'd just rule in favour of the players the first time it comes up. If they want it as a silencer with the prerequisite of putting it over someone's head, that's cool because the enemy will struggle and make it difficult.

If it's debut was from an enemy doing it to a PC who said they'd yell extra hard to call for help, I'd probably ask for a skill check and say the sound does pass through.

From then on, I'd just keep that ruling.