This was from the web site "Howstuffworks.com"

The most likely explanation for the wave-like noise is ambient noise
from around you. The seashell that you are holding just slightly above
your ear captures this noise, which resonates inside the shell. The
size and shape of the shell therefore has some effect on the sound you
hear. Different shells sound different because different shells
accentuate different frequencies. You don't even need the seashell to
hear the noise. You can produce the same "ocean" sound using an empty
cup or even by cupping your hand over your ear. Go ahead and try it and
vary the distance at which you place the cup near your ear. The level
of the sound will vary depending on the angle and distance the cup is
from your ear.

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.

frank