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