[thesystemis]

on a similar note (and because there's some great links being posted here), this video by walter lewin covers the physics of sound and how it relates to music, it could be good secondary material for someone learning about music theory:

http://mitworld.mit.edu/video/168

what's great is that it's a serious physics lecture, but designed for kids, and there's plenty of funky experiments within the one hour.

[psykotic]

If you want to go beyond the basics, I found this book invaluable:

http://www.maths.abdn.ac.uk/~bensondj/html/maths-music.html