If you have a full inventory up until the point where Porky's mom squashes Buzz Buzz, he will notice that Ness can't hold the Sound Stone right now and won't give it to him. Instead, he will transport it to Tracy, meaning that you don't have to hold the Sound Stone in your inventory for the entire game and it will still record the melodies of the "Your Sanctuary" locations even in Tracy's possession. Normally, the Sound Stone remains in your inventory for the whole game and cannot be held by Escargo Express. If you do wish to have the Sound Stone back from Tracy, you must talk to her in-person.