You might try Online Radio Stations

There are some historic folk recordings online - try:
John & Ruby Lomax 1939 Recording Trip
Max Hunter Folk Song Collection

Many performers offer sample MP3 or RealPlayer recordings on their Websites. Oftentimes, you can find a performer by putting www. before the name and .com after - www.debracowan.com will get you to Mudcat's Debra Cowan, for example. Debra and others have samples of their recordings at http://www.efolkmusic.com/.
Let me put in a plug for my friend Dick Holdstock - you'll find samples at his site at http://www.dickholdstock.com.

Hope that helps.

-Joe Offer-