The sea you hear in a seashell is the room you’re standing in. The shell’s hollow is a resonator: it takes the faint noise already around you, a fan, a road, people talking two rooms away, and amplifies the few frequencies that fit its shape, and those happen to sound like surf. In a properly soundproofed room the ocean goes nearly silent. So everyone who has held a shell to their ear has been listening to wherever they were, rearranged to sound like somewhere far away.