voice signal is not a single frequency and single amplitude. It consist of a range of frequencies . But in female voice, the maximum and minimum of the spectrum(for a particular letter) will be little bit shifted towards higher frequency region than in case of a male.
U can hear a single frequency tone by connecting a speaker at a sinewave output of a function generator(connect a 1uF capacitor in series) at a fixed frequency. But if u add a sinewave of one frequency with another sinewave of different frequency then u could feel the difference.
FOR BETTER UNDERSTANDING, just download this java applet