Q19: Why did Sigma not hear that he had the voice of an old man?

     Maybe it’s because…his processing speed was slowed down. I assume he recognized an old man’s low frequency voice with that of a young person’s high pitched frequency.
     On a side note, the reason why he didn’t recognize Doctor Klim’s hologram voice is the same concept of you not recognizing your own voice when you record your voice, since it sounds different.