There is no such thing. AE only does Stereo. If you need to mix multichannel audio use Audition or any otehr audio editing app.
AE, as Mylenium said, is Stereo or mono only. Surround Sound of any kind needs to be mixed in an audio program and added to the program stream through softward designed for the task. That's no slight to After Effects. It's just the way things work.