<human>: Could you distill the essence of the text into 1-3 sentences?

I noticed a little while ago that my sister uses a different voice based on the situation she's in or who she is talking to, and I'm wondering why and what it means.  

For example, she has a distinctly different voice when ordering food at a restaurant, or when she's speaking with our dad vs. speaking with our mom.  Since I became conscious of her doing it, I hear everyone do it all the time - is this common to others?    

I wasn't sure where to post this - Google brought up puberty and responses to fear, but I can't find any specific information on situations/people etc affecting the sound of someone's voice.  Either it's not noticeable or not interesting, but I'm very curious.

**Is there a reason this happens?  Does everybody do it?  Which voice is the "real voice"?  Does that make certain voices fake and others genuine?  Is it conscious or unconscious?  Either way, how does someone choose or develop a certain voice for a certain situation or person?
<bot>: do people change their voices based on the situation they're in, why do they do this, and does it matter?*