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What camera model are you working with? Does it have 5.1 Surround Sound?
Can you provide some additional details about the type of audio system you're using to play back that doesn't sound loud enough in Final Cut Pro X? Are you playing back directly from the Mac's built-in speakers or connecting to a set of headphones or speakers?
Have you adjusted the volume level and playback device in System Preferences so that other applications have sufficient audio level?