Since that time, I have treasured Sophie's words and come to realize the wisdom in what she said as I try to help my student teachers master the infinite qualities of their voices. I have added to Sophie's advice by reassuring my students that they will get much more enjoyment from their teaching if they see it as a "theatrical art" form in which they can play the part by using their voices effectively and enjoyably.
For example, I tell them to sound curious and genuinely interested when they are asking a question; soften the voice or use falling or rising intonation so that is sounds like a genuine stimulus for thought or search for an answer. I tell them when they want student to follow specific directions to say what they mean and mean what they say. It doesn't really work very well if you want to let children know you are not terribly happy with their behavior if you say it with a big smile on your face.
So although I want my students to develop their teacher voices the idea is a long way from what is traditionally known as a "teacher voice". This is something left over from a Dickensian classroom. Effective teacher voices are inspiring, encouraging, enlightening, motivating, sensitive and theatrical. If they are this, they never need to be "teacher voices".