You can not just export the audio in iClone (this has been submitted as a feature request).
Like sw00000p suggests, render the scene to low quality video, then split out the audio.
You can do this with a tool such as VirtualDub (free). Load the video in VirtualDub, then select Save WAV from the File menu. Now you will have a file with just the audio from your video.
Here is little tutorial that explains it: link.