What has one voice but can sound like many people?
Answer: An echo.
An echo begins with a single original sound or voice, but that sound can bounce off walls, mountains, buildings, or other surfaces and return from different directions.
When several reflections reach you at slightly different times, one voice can seem like multiple voices answering back.
The riddle plays with the idea that the sound appears to come from more than one source even though there was only one original speaker.
That is why an echo fits the clue so well: it starts with one voice but can make that voice seem multiplied.
