http://www.musicofnature.org/fledglings/ Fledgling calls, including Northern Cardinal   This is the site containing the follow up for Hope Batcheller's research projsct linked above. It might be interesting to compare to adult calls.

I also stumbled across this site with descriptions of (mainly) birds with sound files.
Near the bottom of the page I found this: "125. ...brilliant scarlet tanager hunts caterpillars, periodically breaking into song... Because the bird is something of a ventriloquist, one that sounds far away could be directly overhead. Walk past the point from which you perceive the sound to be emanating, then look up. With luck you will see this brilliant scarlet-and-black summer singer."

Swainson's thrush is described as being ventriloquial here http://homeport.tcs.tulane.edu/~danny/seal.html .

Seems there's at least a reasonable possibility you are observing a similar effect with the local Northern Cardinals.

maeve