Whales use their lower jawbone to the middle ear. whereby they can ear clearly while they are in the water. Human ears and dolphins ears they work the same . They receive sound directly from the air.
In similar way dolphins and human ears are placed in basic anatomical location which is same as on the head. Without a medium hearing is not possible.
The medium sound for human is air. Sound travels 343 meters per second in air and 147 meters per second in water. The most effective way of transmitting sound waves is through water.
We can say that the sound will be fast to travel in medium if the molecules are closer.