While doing some research on original sound recordings from the early 20th century, I stumbled across two of the oldest sound recordings restored into audible form today.
The first is a digitally restored Edison tinfoil record from 1878 which you can read more about here: websnap08.lbl.gov/Tinfoil.html and I highly recommend listening to the .wav files linked on the page.
The second recording is even older from 1860 and the New York Times describes it in this article available here: http://www.nytimes.com/2008/03/27/arts/27soun.html?pagewanted=all&_r=0
Sound samples are on that page as well. Highly recommended listening. I was amazed that the quality was that good. It's also a great recording to sweeten your audio restoration skills on a bit :)