FM modulation index calculation

FM modulation index is:
mf = frequency deviation / modulation frequency

So, for a 4kHz modulation frequency, you get mf=1, only if the frequency deviation is 4kHz.
This is unusual narrow frequency deviation for FM.
Just to get an idea, the NBFM system (narrow band FM) have about 5kHz deviation, and FM broadcast about 75kHz frequency deviation

Your question is not clear to me. What you mean with FM modulation of sound voice, you mean the speech ?
I'm not an expert, but I think the speech is not a simple FM modulation.

Dear albbg
i want to design FM modulator and its neccessory to me to calculate the bandwidth of my circuit that it related to FM modulation index(mf).i want to calculate mf of voice of human (0-3khz)