xeno-canto.org, please visit the website for more : xeno-canto, the community database of shared bird sounds from the whole world.
Bjarne Nielsen: Bjarne Nielsen's web med feltornitologiske artikler samt billeder og fuglestemmer
The birdsongs are available online in different formats. In order to listen to them, you will need the appropriate software and browser plug-ins. I recommend Winamp Media Player and the VLC Media Player, both of which can be downloaded for free.