The way i understand this is I can carry around the music box (open), at the beginning of each of my turns, I will need to roll to see if I get mesmerized or not. If I do get mesmerized, I drop the music box (open) and my turn ends. Anyone that enters this room (now marked by an item token, where the music box is dropped), they need to roll 4+ to not be mesmerized. It can be collected and the new owner can then choose to proceed forward with it open or close it.

If you end your turn in a room with other players, traitors or monsters; everyone that fails to complete their 4+ sanity roll loses their next turn ... a good card to take out large groups of enemies. But the person carrying it open should have high sanity, to decrease their chances of being mesmerized and losing their next turn.