The National Jukebox gives you free access to more than 10,000 historical recordings—the iPod of America.
Recordings can be browsed by date, location, and language. Genres lets you sort by category—classical, ethnic characterizations, popular music (with several subsets), religious, and spoken word. You can even figure out what music was recorded any day of the year. Today in history or a new birthday song, anyone?
You can also explore existing playlists or create your own for use in class. Why not have your students pick a few songs for their own playlists, and then describe how the pieces relate to each other in terms of historical context?
Finally, you can peruse the entirety of a digitized copy of The Victrola Book of the Opera, 1919.