OK, I think I answered my own question! What I did was to take the
original clip I'm using and pull it down to the 'audio/music' track.
This turns it into audio only. Then I copied that clip, and undid the
pull down, so I am back to the original a/v clip. Next I went into
the clip menu and muted the audio for the clip. Then I went to the
'audio/music' track and pasted the audio only clip I had previously
copied. Now I essentially have a clip where the video and audio are
separate. Any transitions I add do not affect the 'audio/music'
track. I do have to pay close attention to any timing changes that
occur as a result of adding or editing transitions, but otherwise,
this seems to work. Let me know if this makes sense.
Thanks!