There are a number of ideas about what actually makes the 'wave' sound when you put a shell to your ear. One suggestion is that you're hearing the echo of your heart beating and the blood rushing around your body, in particular the blood vessels in your ear. But that's simply not true, because if you ran about a lot before putting the shell to your ear there would be a definite difference in the intensity of the 'waves' you hear. Why? Well, exercise of any sort increases you heart rate hence the waves would be louder, or more frequent, in time with the faster beating of your heart.
Another explanation is that the wave sound is created by air flowing through the shell, and this may have a little to do with it as the sound becomes louder when you lift the shell slightly away from your ear. However, if you put your ear to a shell in a soundproof room (where there is no ambient noise, but air is still cycling around the shell), the wave sound is noticeably missing. So, it must have something to do with outside noise.
When you hold up a shell to your ear, you block out direct noise to your ear. However, the shell captures any atmospheric noise, which then resonates inside the shell. This resonating chamber needs some noise to work with, but otherwise works regardless of whether your surroundings are noisy or not. However, it stands to reason that the louder the environment around you, the louder the sound inside the shell - as more sound waves are 'bouncing', for want of a better term, around the chamber. These frequencies are garbled by the walls of the chamber and become like radio static to us, as our ear is not finely tuned enough to distinguish every nuance. Thus you get that shhhhhh sound, like waves breaking on the sea shore.
The rushing sound that one hears is in fact the noise of the surrounding environment, resonating within the cavity of the shell. The same effect can be produced with any resonant cavity, such as an empty cup or even by simply cupping one's hand over one's ear. The similarity of the noise produced by the resonator to that of the oceans is due to the resemblance between ocean movements and airflow.
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 visit:-
http://en.wikipedia.org/wiki/Seashell_resonance
http://h2g2.com/approved_entry/A46714665#conversations
