The "golden record" that was attached to both spacecraft contained a variety of information encoded on a gold-plated audio-video disc, with visual instructions for playing back the disc in the unlikely event that the spacecraft is recovered by some distant civilization. Distant civilization? Was this even possible? Yes, in theory, because the Voyagers were planned to be the first human-made objects that would leave the solar system and enter interstellar space, with Voyager 1 now having just left the "heliosphere" in 2004. It will take thousands of years or more for either spacecraft to reach any star system, but if one were recovered by someone in the far future, the mission designers wanted to include some sort of message about who its makers were and where this odd artifact came from.
Of course the Voyager record is really more of a message to (and from) the people of Earth, so a lot of thought went into what should be included to represent as much of humanity's heritage as possible - samples of many languages, photos and diagrams, sounds and music. The book Murmurs of Earth is a detailed telling of what was included and how it was all decided and compiled. It's still a very cool book, and it includes all the images that were on the record.
Some later editions of the book also included a CD-ROM with the actual contents of the "golden disc," but my 1978 first edition hardcover did not have this. All versions of the book are now out of print, but many used copies are available through Amazon and other sources. Unfortunately none of them seems to include the CD-ROM, but as you might expect, much of the information that was on that disc is now on the Web. JPL has a web site devoted to the Voyagers, and a section of this site is devoted to the Golden Record itself. There you can find links to many of the images on the record, and most of the sound samples (though none of the music, probably due to performance copyright issues).
One of the sound samples is called "Music of the Spheres" (.wav file) and is a sound rendition of Johannes Kepler's Harmonica Mundi. Here's what the book says about this (page 154):
Kepler's concept was realized on a computer at Bell Telephone Laboratories by composer Laurie Spiegel in collaboration with Yale professors John Rogers and Willie Ruff. Each frequency represents a planet; the highest pitch represents the motion of Mercury around the Sun as seen from Earth; the lowest frequency represents Jupiter's orbital motion. Inner planets circle the Sun more swiftly than the outer planets. The particular segment that appears on the record corresponds to very roughly a century of planetary motion. Kepler was enamored of a literal "music of the spheres," and I think he would have loved their haunting representation here.
How appropriate for the sphere-trotting Voyagers and for this particular blog!
P.S. Although Pioneer 10 and Pioneer 11 were launched earlier and are also departing the solar system, Voyager 1 "caught up and passed" these spacecraft in terms of distance from the Sun and reached a distance of 100 AU (astronomical units, i.e., 100 times Earth's average orbital distance from the Sun) in August 2006, and is now in a region called the heliosheath, a zone where the sun's influence is very weak, though this is still not considered "interstellar space" (that will take about 10 more years).