I always thought that these were the melodies of the waves. But River Horse has yet another they're coming out with. This Melody of the Waves is a new adventure for the Tails of Equestria RPG. So, your ponies better get their sea legs and maybe pick up some little water wings. It's high-seas adventure ahoy. You can pre-order your copy now.
From the website:
Welcome to the Singing Sea, an enchanting place of myth and magic to the west of Equestria’s shores. Legends tell that those who sail there sometimes hear delicate songs drifting up from the waters, telling of ancient exploits from the days before Equestria.
These days, the Singing Sea is not so calm. Ancient magic seeps from places of power across the Singing Sea, creating strange phenomena such as the unending Everchill Mists or the sprawling Sea Forest in the south. It across these waters you must sail if you are to uncover the mysteries that await, and prevent a cataclysm that could affect everypony who calls the Singing Sea their home!