Literally, your voice is the sound coming out of your mouth. Metaphorically, voice can also mean the way people express themselves.
People have high voices, low voices, quiet voices, and loud voices — but the concept of voice goes well beyond people talking or singing. Writers talk about finding their voice, which means they've developed a unique identity or style. A lawyer can be your voice in court, where he can speak to the judge about your concerns. If you are shut out of some process, then you have no voice in that process.