However, this theory doesn't hold true in a soundproof room. In a soundproof room, there is still air, but when you hold the seashell to your ear, there's no sound. The most likely explanation for what causes the wave-like noise is ambient noise from around you. The seashell that you are holding just slightly above your ear captures this noise, which resonates inside the shell. The size and shape of the shell therefore has some effect on the sound you hear.
Different shells sound different because different shells accentuate different frequencies.
You don't even need the seashell to hear the noise. Go ahead and try it and vary the distance at which you place the cup near your ear. The level of the sound will vary depending on the angle and distance the cup is from your ear. 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.