James Cotton - Cotton Crop Blues, on Sun Records. Recorded 1954, with a guitar solo harder/heavier than most records up until the late 60s (with a few exceptions like Dick Dale). And it features distorted power chords. Pretty much the first really heavy track out there, I figure. It's my go-to point to show people the origins of heavy, distorted guitar music.
I also like to show them some Bob Wills & His Texas Playboys, as he had a lot of electrified instruments (electric guitar, electric mandolin, pedal steel) and was a big influence on Chuck Berry and Elvis. Plus I love Bob Wills.
http://en.wikipedia.org/wiki/Origins_of_rock_and_roll does a pretty good job of covering the important records that led up to the explosion of rock music in the 60s. Not sure about how it is beyond that.
Pretty glad a musical topic came up here on HN, though. I'm more passionate about music history than I am about computer science (though there are no career prospects there, so CS it is).