When I was a kid I was told that big shells made noises when air was blown
in a certain fashion through a broken part of the apex. What I was taught
later in my physics class was that smaller shells can have noise resonate
through them but the pitch and frequency is too high for humans to hear. I
tried this out on my poor unsuspecting cat and I thought she was going to kill
me. I might add that my lips wern't exactly pleased with me. So I dont
know if your character has super hearing but the possibility of a whistle or
horn made from a shell that can be worn around the neck is probably not
highly likely ( though I may be wrong).
Sarah Watson