I don't know whether you have tried this but if you have never then it is true that when you place a seashell over your ears you will hear some sounds of ocean waves coming out of the shell. But where does that sound come from?
Is it the real ocean wave sound that is confusing your ears? No. Because even when you move away from the beach with the shell you will still hear the sound. Does the shell record the sound? No. It has no ability to record the sound.
So where then does the sound come from?
Well, scientifically according to livescience.com, the shapes of the shells make them great amplifiers of ambient or too fine sounds.
They make the sounds even louder. When air gets into the shell's opening, it gets bounced around by the hard walls of the shell's inner curved surfaces. The reverberating air, therefore, produces the sound similar to the sound of an ocean wave.
The volume or pitch of the sound depends on the size of the shell. Air takes long to bounce back and forth in a bigger shell than it does in a smaller one. For this reason, the smaller shell will produce sound with a higher pitch than a bigger shell.
This is similar to when you place a cup over your ears. You will listen to the same sound, a sound you will always hear when you are relaxing at the beach. The same explanations apply.