If you're new (or maybe not-so-new) to birding, you should know about these resources:
Todd Wilson has an excellent blog devoted to the bird song recordings he has made - it's called Bird Voices, and you can find it here.
Of course we also have the great work done here in WI by John Feith; see the audio links at the WBBA site at this link.
If you want to range outside our borders in terms of song and bird sounds, many experienced birders know about Xeno-Canto - see it here. And Cornell's All About Birds song pages are linked here.