Singing Seashell is a magic tool described in detail in Monster Girl Encyclopedia World Guide - Side III book.
Book Entry
Oh yes, I completely forgot! Yes. Here's my song, please listen to it carefully! It's the Singing Seashell, a magical tool that is made with magical seashells out of mamono realm and is able to play music. The song begins to play when a magic sea pearl with a song is placed inside the shell.
Isn't this a breakthrough technology for Court Alf? I can tell you that the Singing Seashell is now attracting the attention of other mamono realms. In addition, Court Alf also has a smaller portable player developed on Saida as well as headphones that use paired clam shells as counterparts to this magical item.
So yes, I will give you this magic sea pearl with my song in it. Can you get a little excited about that? It's the same pearl that I recorded for the first time, a memorable first pearl! You see, it will soon be the song of the best Song Maiden, no, you possess the first pearl of the future Diva herself! Indeed, you are the luckiest man in the world! From now on, every day, every night, you will be able to listen to my beautiful singing voice at your bedside!
Eh? You want to hear my singing voice directly? What are you talking about!?
So you want to listen to me sing, every night, that's, uhm...what am I supposed to do now, huh!? That's what it means!? Wow, I've never been told that, what am I supposed to do? But thinking nothing of it, I wish you would have decided to do it after hearing my song at the concert... However, hmmm...♥[1]