The Voice Box is an invention made by Romeo for use in "Speak UP, Gekko!". Its primary purpose is to use a ray to take the voice away from its victim. Romeo used it to take Gekko, Owlette and Catboy's voices and pretend to be them, saying false things such as "R-O-M-E-O, Romeo!", and how "awesome" and "smart" Romeo is.
This invention backfired on Romeo in the end of the episode, taking his own voice.