that's not exactly true. The human voice is not a simple tone, but is made up of a fundamental -- think of it as the foundation tone -- with harmonics layered onto it. If you played back a recording of a person speaking and slowly filtered out frequencies starting from lowest to the highest, you would still be able to hear what was being spoken pretty well until you got into the upper frequency ranges (above 1500 kHz). The human brain does a good job of recreating the fundamental from the harmonics.
The telephone, for example, transmits a very low-fidelity signal (limited to between 200 - 3400 Hz) that is still completely intelligible.
Naturally, you wouldn't want your center channel to miss out on any portion of the human voice, but sounds below 80 Hz do not contribute to the intelligibility of human speech and can be safely diverted to other channels or the sub.
