The music box was an item that appeared in Act 35 of the live-action series. It was created by Zoisite and given to Sailor Venus to use. Its purpose was to erase Usagi's memories of the one she loved, but according to Zoisite, it had to be used by one she trusted.
As Minako watched Usagi's memories of Mamoru being erased, however, she realized that the two of them really did love each other. Overcome by guilt and remorse, Minako dropped the box and allowed it to shatter on the floor, breaking the spell.