i think a voice password is a bad idea in almost all situations.
if the room you are in is noisy, and your mic picks up theis ambient noise while you are verifying your voice, you may be locked out.
best would be a fingerprint scanner on a laptop trackpad or in the spacebar, that way it is convienent, and will be less of a problem.
but, my guess is no, to alter how os x asks for your administrator password seems to be a risky idea, to all but the most experienced programmers. you could accidently lock yourself permanently out of the machine...