SonicFit has lessons and exercises to teach music literacy fundamentals and theory, and develop aural and sight singing skills. Lessons and exercises are FREE. Development is sustained by the fee-based databasing of student progress- an optional component.
A great place to start is the scale degree exercise, at the top of the list of exercises under sonic comprehension, as it is the corner stone of my pitch pedagogy. For music fundamentals, just start with the first lesson.