Cornell (http://www.birds.cornell.edu/) is famous for its collection of audio bird calls, songs and squawks. You can get them in various forms, including printed sonograms (frequency vs time.)
Expert birders do c.90% of their identifying by ear. I began to try to train my ear after too much time wading in malaria-infested swamps. Whenever I look through binocs., my mouth drops open for some reason. So, I swallow a lot of bugs.
It is very helpful to spot the bird while he is singing. If you are in the NE, you have a good window of opportunity until early May when the trees leaf out.
Start with the easy ones; the scream of the red-tailed hawk, the wolf whistles of the cardinal, the gorgeous flute-like songs of the thrushes and vireos, the who’s of the barred owl, the caws of the crow, the croaks of the raven, the white-throated sparrow, the ovenbird, the robin, the chickadee, the nuthatch, the melody of the gold finch. the sewing machine noise of the chipping sparrow ..well, you get the point.
Here’s Robert Frost’s sonnet about the ovenbird (familiarly known as the teacher bird, due to its song).
http://www.bartleby.com/119/9.html