Chris Juried’s History of Recording website is well worth a look.
Rick Chinn\’s Uneeda Audio site has an eclectic collection of useful audio information once you start digging into the arcane details. This includes things like pads, phantom power, M-S recording, and even how to solder artfully!
A history of microphone connectors can be found here. This includes the XLR connector.
Chace Productions in Burbank has posted an excellent glossary of sound terms (slightly skewed to the film production end of the business).