In the Audio Device Options window of the Audio/Video Settings, the number of audio channels available is fewer than the actual number of channels that the audio interface provides.
For example: A 10-channel audio card might report a separate stream for each channel, leading Final Cut Pro 4 to show the card as only being capable of a single channel.
Solution
This issue only affects the setup in the Audio/Video Settings window. If you set the sequence settings (choose Sequence > Settings and click the Audio Outputs tab) to support all the channels the device supports and configure the output map accordingly in the sequence settings, then all channels will be played to the device.
Note: With this setup, Final Cut Pro 4 will erroneously warn you that you have set up the sequence with more channels than the device actually has. This warning can be ignored.
Additional information
Final Cut Pro 4 works with most multi-channel audio interfaces that provide a Mac OS X driver. While Final Cut Pro 4 expects audio interfaces to report a single stream containing all available audio channels, some interfaces report multiple streams. At this time, the Audio/Video Settings window presents these devices as only being capable of the number of channels in the first stream.