Everyday
Can you really hear the ocean in a seashell?
The short answer
No. The sound is the ordinary background noise around you, caught and amplified by the shell's curved hollow. The louder the room, the louder the ocean. A cupped hand or an empty mug over your ear does the same thing, and in a soundproof room a shell goes silent.
Hold a shell to your ear and there it is, the long hush of waves. It is a lovely effect and it has nothing to do with the sea. A shell is a hollow with a narrow opening, and any hollow like that resonates: it picks out certain pitches from whatever sound is around and makes them louder. The ambient noise of a room, the hum of a fridge, traffic, your own breathing, gets filtered into a soft rushing roar.
You can test every part of this. Different shells sound different, because different shapes favour different pitches. A mug works. Cupping your hand works. Take the shell into a quieter room and the ocean gets quieter. Take it somewhere perfectly silent and it disappears.
People sometimes say the sound is your own blood flowing, which would be a good story, but it is not true either: the sound does not get louder after you run up the stairs. It is simply the world, heard through a shell. The sea stays where it is.