Noise from outside the shell also can change the intensity of the sound you hear inside the shell. You can look at the shell as a resonating chamber. When sound from outside enters the shell, it bounces around, thus creating an audible noise. So, the louder the environment you are in, the louder the ocean-like sound will be. For more information check out the links on the next page.
Do you remember trying this as a kid — Holding one of the seashells you grabbed as a souvenir up to your ear? It seems like no matter how far away from the ocean you are, you can hear it.