Russell,

"is there a way to find an eq magnitude that would correspond to the oscillation of middle C?"

Put more broadly, your question could be "what factors determine the frequencies of earth motion in an earthquake"? And, why are these frequencies in the 10-30hz range (as Roger noted)? Diane mentioned building resonance and there is ground resonance too. But what factors determine the frequencies right at the source (the rupture)? Presumably a very small rupture would yield higher pitches?

This USGS site has some sounds, but it is mostly knocking, clicking and rumbling.

http://quake.wr.usgs.gov/info/listen/allsounds.html

Another form of listening is:

"In this case the coverage consisted of Navy hydrophone arrays (listening for
quakes rather than subs)"
http://www.aip.org/pnu/1999/split/pnu459-1.htm

More on hydrophones:

"During the past 12 years, researchers at Oregon State University's Hatfield Marine Science Center have recorded more than 30,000 earthquakes in the Pacific Ocean off the Northwest coast - few of which have ever shown up on land-based seismic equipment.

The earthquakes, most having a magnitude of 2.0 to 4.0, originate where the Juan de Fuca plate, which is creeping inland, is gradually is being subducted beneath the North American plate.

"During a test, we exploded a stick of dynamite below the surface and it didn't register on land at all," Dziak said. "Yet hydrophones recorded it as a magnitude 1 event 6,000 miles away. That's why the hydrophone array is so important to our research."
http://oregonstate.edu/dept/ncs/newsarch/2004/May04/hydrophones.htm