A command voice is a voiceprint-based control system for certain Sparky creations, based on certain theories of vocal harmonics. It appears to allow for control of the body, but not necessarily the mind; someone appears to be prevented from speaking, but not necessarily from wanting to speak. However, there are also cases where the command voice can affect the mind more deeply; some dialogue has implied that the right commands . 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 . However, this power is not absolute. Anevka Sturmvoraus apparently believed that Lucrezia's creations upon the first person who issues a command to them with a suitable command voice. In that case, only the person who left this imprint on a thrall will have truly complete control over that thrall.
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 possibly 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.