What does "modulate your voice" mean?

A. Vary the properties of

B. Speak in a quieter tone

C. Raise the volume to high

D. Be perfectly silent

The right answer here would be A - the phrase 'modulate your voice' refers to a need to vary your voice's properties. Modulating means changing the pitch or tone, and can be used referring to one's voice, or to an instrument. You can modulate both up or down, and this usually involves changing the key signature in which you are playing or singing.