const user = {
  profile: {
    settings: {
      theme: 'dark',
      notifications: null
    }
  }
};

const getNotificationSound = (user) => {
  return user?.profile?.settings?.notifications?.sound ?? 'default';
};

console.log(getNotificationSound(user));
console.log(getNotificationSound(null));
console.log(getNotificationSound({ profile: {} }));