A person's voice was the sound they produced when they spoke.
When The Doctor was imitating various people on the USS Voyager, he was having trouble imitating Chakotay's voice, so he claimed to have a scratchy throat. (VOY: "Renaissance Man")
The Doctor found Seven of Nine's voice aesthetically pleasing. She stated that it was probably due to her vocal subprocessor. (VOY: "Someone to Watch Over Me")