According to Brant Gardner, the most suspect part of Nephi's disguise was certainly his own voice. In the dark, wearing the armor of Laban, he might be mistaken for Laban. But would that mistake be repeated when Nephi spoke to a servant who would surely know? . . . Yet Nephi endeavored to speak, and found that he spoke with Laban's voice. That experience would have even further supported his understanding that the Lord was behind his actions. [Brant Gardner, "Brant Gardner's Page, Book of Mormon Commentary, [http://www.highfiber.com/~nahualli/LDStopics/1Nephi/1Nephi4.htm], p. 7]
Note* It should also verify to the reader that, symbolically, Nephi had been given the authority (or voice of Laban) as far as the records of Joseph's lineage were concerned. [Alan C. Miner, Personal Notes]