Hello Tweeters,
I have several apps on my Iphone. The one I like for bird songs is called BirdTunes. Not a field guide but a bird song guide. It has more songs and calls than the usual field guide. In many cases they include the "standard" call of many birds but will also have chip calls of some of the warblers as well as calls of other birds that aren't normally included in the field guides. They list what state the birds where recorded in so you can take regional dialects into account. It is amazing how much some species sound different in other parts of the country.

Neil Zimmerman
Brier, WA