Cymatics, the study of wave phenomena (often of material media on a vibrating substrate), was popularized by Swiss MD Hans Jenny (1904-1972). Though often discussed in the context of "new age" ideas, DIY techies and artists have expanded interest in the phenomena.
Below is a recent TED Talk from Evan Grant, Making sound visible through cymatics. It's nice to see an old idea get some high profile exposure; Hans Jenny's '70s 2-volume book Cymatics had been hard to find, but was reprinted awhile back, and the film narrated by Jenny is now on DVD (pieces are on YouTube). Cymatic figures are more often called Chladni patterns, especially in acoustic and physics texts, and are used to study nodes and vibration in instruments like violins and tablas. Someday sound will be important in providing cues in visualizations.
Video Thing mentions that a DVD of "light waves" artist Richard Baily (Solaris, Fight Club, Superman, ArtBeats, etc) will be available through the Center for Visual Music. His passing last year was noted here last year, and there are more resources at CMV.
In After Effects, ever since the now defunctMotion Math, you can synchronize any parameter with any other parameter, so a range of audio effects can be tied to graphic properties to create visual music. Visual music has a long history in media like color organs, film and abstract animation, light shows, CGI, installation art, and even cave art. Abstract animation is not just eye candy, but often attempts to communicate or stimulate synesthesia or mystical states. While forecasts for an expanded or synaesthetic cinema (PDF) haven't quite come to fruition, motion graphics has. The term seems to have been coined or at least popularized by visual music artist John Whitney, who pioneered motion control cameras and the slit scan technique (showing it to Trumbull and Kubrick) -- and in 1960 named his company Motion Graphics, Inc. SIGGRAPH has a peek at some of his movies.
Later, Vice featured Whitney too. Here's some of his work, along with Lapis from his brother James Whitney.
Greg Jalbert, author of Bliss Paint (for Mac, RIP) collected info on some artists at Tonecolor some time ago -- including the rarely mentioned "Raster Masters" who used to provide incredible live visuals at concerts in the Bay Area. Some say that the genesis of Trips Festival and rock music light shows was the Vortex Concerts at the Morrison Planetarium in San Francisco, organized by visual artist Jordan Belson and composer Henry Jacobs in 1957–59.
I wanted to check out the new YouTube embed player, where the menu button reveals a Dock-like UI for related movies. So here are video demos of Chladni (intense example here) and cymatic patterns showing how a sound vibration effects media (powders) through a substrate. The form the sound takes depends on the nature of the media and reveals the nodal structure of the substrate from which particular tones emerge. Chladni patterns are still used in in musical instrument research; the best cymatics examples don't seem to be on the web right now, except a sample of Han Jenny at bottom.