I have seen many guys asking how to export audio metronome.  Why not make things easy:
Audio metronome could appear in the track inputs, just like soft synths do. To keep it hidden for users that don't need it there should be a checkbox in Preferences/Metronome: [ ] Show Audio Metronome in inputs, off by default.
 
That's what I would do if SONAR was my creature. :-)

Great point, Pan.  I hope Cake listens to you.  I have been painstakingly making audio track metronomes since Sonar 2.  Of course I have to slice up the clips in case I decide to change the tempo! 

Use a Midi metronome and you'll never have to slice again, irrespective of tempo changes

I use Session Drummer for this, but anything will do - even the TTS-1