I once worked with a young girl with Selective Mutism that, when in pain, would actually open her mouth to cry with tears pouring down from her eyes, but without making a single sound. I worked with her for months in her school setting and never heard her speak -- ever. One day her mother brought in an audio recording of her at home while she did not know she was being taped. I was surprised to hear her delightfully chattering, laughing and, basically, non-stop talking without reserve. Recently, I worked with a young boy who could not speak out loud in public, but instead, used a whispered voice. The constant whispering was actually damaging his vocal cords, leaving him with a raspy voice once he did speak. After months of therapy, he finally was able to resolve the problem and could use his normal voice in familiar settings but continued to use his whispered voice, or no voice at all when in an unfamiliar or stressed situation. 

A rare disorder, Selective Mutism is found in less than 1% of the population and typically occurs before the age of five. The disorder may subside in a few weeks or months, or may persist for years, depending on the child. Treatment is difficult because of the deep emotional roots of the problem.