There was an interesting documentary on TV a couple of years ago about a
group of scientists who were given the old US Navy underwater listening
network for a few months before it was shut down. Amongst other things
they used it to track and listen to whales around the world, and one of
the tentative conclusions they reached was that the 'songs' were long-range
sonar for navigation in the deep ocean; because of the inversion layer
they could potentially image the entire ocean and the differing 'songs'
allowed them to distinguish their returns from other whales. Wish I'd
taped it, but I didn't realise how interesting it would be until afterwards.
Personally I find the idea of animals who can 'see' an entire ocean far
more impressive than the idea of animals singing to each other, but I
guess it takes all sorts...