I'm kind of late to the game in this thread, but my thought about theory is that it should start with physiology and technology. At each level we're reminded that we study the aspects of music that people have already invented, and that we may overlook a lot of important things, such as rhythm.<p>Physiology: Some of this may be speculative, but it seems likely that "harmonious" intervals, that have a superposition of harmonics, have a physiological effect.<p>Technology: The 12 tone scale could be described as a technology for tuning an instrument with harmonious intervals. Temperament is a technology for solving the problems of tuning primarily keyboard instruments.<p>Naturally, math is involved in understanding these things, as with many areas of science and technology.<p>I would talk about a handful of widely used instruments, such as keyboards, strings, winds, guitars, and drums.<p>Then you can begin to talk about scales, chords, melody, form, and so forth.