Google Now seems pretty random when it comes to notifying you of new cards. At seemingly random times throughout the day I will pull down the notification shade and have one from Google Now that says "3 cards", or something to that effect. No notification sound ever plays unless its a traffic update, or an emergency alert. It would seem to me that the only way to know when a new card pops up is to manually check Google Now, or your notification shade. There are apps that will play a sound for every notification you get from any app, but I don't think Google Now produces notifications as each card is generated so even that wouldn't help.
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