Whether or not the voice was precisely that of Laban, the entire effect was so certainly Laban that the servant was able to accept Nephi as Laban without hesitation. Even if the voice were somewhat different, the servant's expectation that it was Laban would have created a condition where he heard what he expected to hear, and acted accordingly.
"How Nephi disguised himself in the clothes of Laban and tricked Laban's servant into admitting him into the treasury is an authentic bit if Oriental romance, and of history as well. One need but think of Sir Richard Burton's amazingly audacious masquerades in the East, carried on in broad daylight and for months on end with perfect success, to realize that such a thing is entirely possible." (Nibley Lehi in the Desert . Bookcraft. 1952. P. 117).