Why in some video games where i create my own character why do they sometimes doesnt talk or have a muted voice?

Usually it is a design choice that gives the player the feeling that they are that character. So when your character speaks, it has diolog options of what to say. Esentually you are reading it in your voice, and is to a point an extension of you. It makes it feels more like you are creating your own story and choices. While if the character did have a voice it would be more like you are controlling someone else.