There are people who sing a melody with a musical instrument. They hear the notes and repeat. When they take away the insturment, eg piano or guitar, they cannot sing the melody correctly. What was it that went wrong when they practiced with the guitar or the piano? Did they just repeat notes rather then learn how they feel and sound?
Why doesn't the standard way of teaching how to sing work for some people? Is just repeating what you hear someone or yourself play really what learning to sing is all about? This seems to be the most popular method for all styles of singing.