Mr. Boxy
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While people say they hear the ocean, the sound has nothing to do with the shell being near the sea. If the shell really did capture the sound of the ocean you could bring a shell to a concert and something like this would happen:
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What you are hearing is amplified ambient noise. The shell acts like a resonance chamber: As sound waves enter the shell they bounce off the surface with little loss in volume. This creates feedback, increasing the volume of this noise while the echoes from this bouncing smooths out the sound. This is not unique to shells: Pottery contains calcium carbonate, the mineral that gives shells their reflective ability. If you press a coffee cup to your ear, you'll hear the same sort of sound.
Resonance chambers do have practical uses. Concert halls use reflective material around the stage to reflect sound into the audience. This isn't perfect with "dead spots" that lack this resonance. This is a major headache for sound techs, as a performer in one of these spots will be much quieter than those that can take advantage of the resonance effect. Resonance chambers are also used in engine exhausts to redirect sound waves away from the exhaust valve to reduce backpressure. This improves power and gives the engine a smoother sound.
 Posted 5303 day ago
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