On this site you can create and save soundtracks of four different sounds that include nature sounds (birds, sheep, crickets, etc.) as well as man-made sounds (tribal drums, children's giggles, and even Darth Vader!) I personally go for the birds by the babbling brook myself, but I think it will be a fun way for my kids to explore sounds by making their own soundtracks and to fine tune their listening skills by making soundtracks for them to listen to and figure out what they are listening to.
While I was playing around, I made a track that I could fall asleep to night after night and I called it Forest and Farm. Enjoy!