Even though the speaker only refers to the seashell once, it still strikes us as super-pretty and powerful in the way it can soothe maggie's troubles away. In that small shell we can hear the music of an entire ocean just singing away. Symbolically, maybe the singing shell represents all of the things we find comfort in, whether that's a song or just a soothing place or object.
Lines 3-4: Once that shell starts singing, maggie can't remember all of the stuff she was worrying about. It's like she's hypnotized by the ocean's sounds that remind her it'll all be okay in the end. There's always something to help soothe our worries in life, and those things seem to be easy to find out in nature.