Mine doesn't just sing, he will play his recorder at home while he works. Of course that doesn't work if he is writing and he can't and doesn't do it at school. He always seems to know he couldn't do this as school and it was usually not a problem. The time it was a problem was in 6th grade, he was having major problems with anxiety and he would start muttering the more anxious he got. Working with a psychologist and learning different ways to focus that anxiety helped.