I recently needed to do just this...
But I found it easier to simply copy the audio track to
the "narration and music" track, MUTE the audio on the
original video, and then "slice" in the cutaway video.
The trick here is making sure the "cut out" video and
the "inserted" video are exactly the same duration -
otherwise the audio will be out of sync. (Note this also
means you can't add a transition, as WMM "pulls" video
into the transition - which would also make the audio out
of sync with the subsequent video.)
Good luck!
~jon ashley