Voice modulation isn't used, because of the bandwidth requirement. Voice range frequencies for radio purposes start at 20 hertz and extend to 2500 hertz. The, so called, high fidelity range extends to 20,000 hertz but most of the important voice energy required for communications is contained in the region under 3000 hertz. Transmitting voice over the carrier therefore requires a higher bandwidth than CW transmission, and as the frequency range allocated for ADFs is limited, transmitting voice would limit the number of ADFs within any given area.