I struggled with this one at first too, since you can hear Ishmashell long before you actually find him, and there are lots of other shells on the beach. Keep exploring and eventually you'll find him. And you'll know it's him.
First time I played I thought it was my wizards voice and I thought he sounded really old and talked alot, and it didn't fit because he was young and strong and such. So I rerolled to take rogue voice I think it was and then found out it was the seashell talking