I looked for music theory classes on youtube. I found a lot that looked bad, but one I found worth perusing[1]. (A word of warning. It is really corny, but I find him adorable).
For sound synthesizes I've had an easier time (I just wish I could get my hands on some analog modules). I've been reading Gordon Reid's Synth Secret[2], recommended often here on HN.
And if you know javascript and the DOM I do recommend the docs on the Web Audio API, both on MDN[3], and the W3C spec[4] and implementing the modules described in the Synth Secrets. The API maps very nicely (at least in the eyes of a novice) to how modular synthesizers work.