
Your voice is the second most important element of communication -
Body Language
Dr. Albert Mehrabian, currently Professor Emeritus of Psychology at UCLA, is best known for his publications on the relative importance of verbal and nonverbal communication - otherwise known as the 55-38-7 rule. In other words...
55% of the message we convey to other people is conveyed through Body Language
38% is formed through the Tone of your voice
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7% only - is conveyed through Words
From the picture... do you believe the message conveyed by the person is more likely to be positive or negative? Whoever she's talking to can't see her - all they have to go on is her tone of voice (she's smiling, so likely to be appealing) and the words she uses ( can't hear her but a subjective judgement would lead me personally to believe that the interaction with the customer is going to be pleasant and positive)!
The same applies at interview - your voice is a powerful communicator.. Use it positively and you'll be well on your way, use it negatively and you almost certainly fail to be selected.
VARIETY OF VOICE
So what makes up your voice? Four elements...

The pitch of your voice can be a significant determinant in how you are perceived at interview. A lower pitched voice is generally regarded as being more attractive, conveying a picture of someone who is affable, influential and easy to listen to. A high-pitched voice conveys someone who is excitable, tense or anxious - qualities you don't want to portray! Practise deep breathing while waiting for the interview to begin.

This is about inflection and emphasis NOT VOLUME! Try saying these words out loud...
Hello? Hello. Hello!
The word is the same in each case but the meaning conveyed is completely different. There are many sources of information regarding exercises you can do to improve your use of inflection. Take a look and see what you can learn. Using inflection conveys the power you want without the volume. A must to avoid is speaking in a monotone. Implies that you're not enthusiastic or passionate about anything - including yourself.

The key to speaking at an appropriate pace is remembering that you need to speak at a rate that allows the interviewer to understand what you’re actually saying. Listening is a two-step process - you have to physically hear what is said and then translate what you have heard into meaning. If you speak too quickly, this vital step of the process is lost. Unless you are a naturally even-paced speaker, slow down and don't come across as excited. Enthusiasm at an interview is fine - excitement is not.
and lastly...

The use of the pause is an art which must be practised. But many people are not comfortable with silence and so fill the gap with meaningless words which only serve to weaken their point or reply. You may find yourself at interview searching for a word. Don't blather with fillers - pause (it may seem like an eternity), breathe in and you'll find with practise that you think faster and can retrieve the appropriate word.
Be as expressive as possible when you speak and remember to smile. Speak in short sentences. Be focussed. Use silence to your advantage. Use it to think ahead; use it to give the interviewer a break from listening.
Use the 4 "P"'s to your advantage. They can help you win!