It's usually easier to add the music or the narration in one pass and then
add the other one in another... saving as DV-AVI with each pass so as not to
lose quality.
But you can overlap clips on the audio/music track... so if you have a 4
minute narration and 4 minute music file, you can pretty much overlap the
two fully... if they play back as you want.
--
PapaJohn