• If you want your audience to quiet down, to be calm or to perhaps hang on your every word, speak more quietly.
• f you want to rile up your audience, speak with a stronger voice. You can also use volume to convey your passion.
• Try not to speak too quickly. Most audiences will have trouble following along. Instead, speak at a moderate pace. Slowing down your speech even more will emphasize each word.

You modulate your voice when you change pitch, like when your voice goes up at the end of a question. 
Just like with music, its rhythm and tone can convey a meaning that may be different than the literal interpretation of your words.

Being able to modulate your voice correctly...