An output device is any device you use to convert FCP's data stream to some format of video, such as DV, SDI, analog composite, or component. Any time you send video to a monitor other than the computer monitors connected to the graphics card of your Macintosh, you are sending to an output device.
A few common factors prevent video or audio from being sent out to your target output device. These factors range from device-specific settings on the output device to FCP's internal controls and toggles for A/V output.
To find the solution to your missing-audio or -video problem, first determine whether both types of output are missing or just one, and whether the output is partially missing or completely absent. Then consult the "Missing Audio and Video Problems at a Glance" table, and find the symptom description most similar to what you're experiencing. The table will point you toward the possible cause of your problem and the section you can read to find out how to fix it.