One aspect of a speaker's use of voice is the highness or lowness by which a person speaks. by what term is this referred to?

The term that refers to how high or how low the speakers voice is would be pitch. This is a key element to consider when giving a speech. This is because pitch is directly correlated to mood, as well as tone. A higher pitch can signify cheer, as well as fear, and possibly sadness. A lower pitch may also signify sadness, but will also signify anger or serious demeanor.