Dolphins and whales are related. When dolphins are sleeping they show this. In the day they are quite unlike whales. Dolphins show off by jumping, swimming and catching balls. At night, however, they appear to "speak" a whale language.
The sounds they express closely resemble the humpback whale song. They only do this after they have heard the song. The song was in a soundtrack played during a performance for the public. This is an indication how all mammals deal with new things. The brain works on new things below the level of consciousness.
So well imitated were the whale sounds that tests done on them by volunteers showed people thought they were real whale sound. Dolphins mimic other types of sounds during the day, but they tend to zero in on whale sounds at night showing the evolutionary link.