Tone of voice
There are many aspects to the voice such as the timbre, rhythm, loudness, breathiness and hoarseness of our voices. Some people have a more calm, warming and comforting tone in their voice, those are the kinds of people that are more relaxed and confident in their daily lives and don't have many things to worry about. Others have a more worried, cold, and anxious voice because of what they have to go through in their lives, many of those things are very stressful.
Tempo of voice
Some voices have a fast paced voice, and others are slower. If it is a faster voice then that person is either nervous, or in a rush. If it is a slower person, then that person is more calm and relaxed and confident in what he/she says.
Clarity of voice
The clarity of the voice can determine of people would even consider you for various things, such as a job. It is what people look for as well because they don't want to keep hearing an unconfident person, and you surely wouldn't want to keep repeating yourself.