A hearing protection earplug having a shell worn at least in part in a user's ear canal, an active unit having an acoustic output transducer and an arrangement for producing output audio signals for said output transducer, and a detachable connection of the active unit to said shell, the shell including a sound passage extending between an outer opening of said shell and an inner sound output opening of said shell and having a valve mechanism which is moveable, upon connecting said active unit to said shell, from a closed position acoustically closing said sound passage into an open position acoustically opening said sound passage, said valve mechanism being biased towards said closed position when said active unit is removed from said shell, and wherein said output transducer is acoustically connected to said sound output opening via said sound passage when said active unit is connected to said shell.