http://artsites.ucsc.edu/EMS/Music/table.html – UCSC Electronic Music Studios Articles on Acoustics, Digital Audio and Electronic Music. So many good articles here.
http://www.purveslab.net/seeforyourself/ – Purve Labs “See For Yourself” section. A couple of good audio demonstrations here on harmonics.
http://music.columbia.edu/cmc/MusicAndComputers/ – Music and Computers; A Theoretical and Historical Approach. Wow, so much great information here about sound theory, recording theory, analog to digital conversion, Fourier transforms, and a lot more. (Some of it gets a bit technical, but it’s all very clearly described!)
http://www.reddit.com/r/audioengineering/wiki/index – Audio Engineering Wiki on Reddit (easy to read and understand, and lots of good useful information.)