Answer: They would have learned to sing
There are two solo parts to the opening song in the film, which allow the dolphins to say thir goodbyes and thanks to the human race. The first is sung by a younger sounding dolphin; "If I had just one last wish, I would like a tasty fish", and this is followed by a much older sounding dolphin, who then sings; "If we could have changed one thing, we would all have learned to sing."
This song is used as a brilliant introduction to the film, as not only does it show that the world is about to be destroyed, but it also points out humankind's ignorance of this event.