A command voice is a voiceprint-based control system for certain Sparky creations. It appears to allow for control of the body, but not necessarily the mind; someone appears to be prevented from speaking, but not from wanting to speak. This system is most notorious for its function in the control of revenants, but that is far from the only application we've yet seen.
Lucrezia Mongfish seems to be the most extensive user of voice control that we know of, having incorporated it into Von Pinn as well as the slaver wasps and having to command the . Her daughter has a voice similar enough, especially while in the madness place, to command Lucrezia's creations as well, which fact has been a on .
Aaronev Sturmvoraus took a in the command voice. His son, Tarvek Sturmvoraus has also in this technique, not only by using his sister Anevka's clank voicebox to counterfeit the Mongfish voice for use on revenants, but also by imprinting Anevka herself to his own voice in case of need.
The command voice has likely caused someone to die by suggestion(albeit without the speaker's awareness at the time) , and has deliberately been used to issue such a self-termination order as well, though its success in the latter case has not been confirmed.