Lightnote is simple, clean, easy to use and, more importantly, easy to understand. Free lessons cover sound waves (why notes in particular and sounds in general exist), harmony, the pentatonic scale, the chromatic scale, chords, keys, and diatonic chords. Premium access includes guitar theory, piano theory, and more. Side note: some people say code is poetry, but as long as spell-check doesn’t recognize “pentatonic,” I’ll continue to have my doubts.
How To Mix Music by Landr.com covers the basics of mixing with theoretical and practical guidance.
This post on Spinditty covers creative chord progressions and cadences.
How to Construct the Perfect Jazz Solo. Funny because it’s true.