I can't find instructions on paste/replace (one to one, not ripple)
my specific interest is replace frames and leave audio untouched
'preciate it :-)
FlashBack does not allow frames to be replaced - you would need to separately insert the new frames and replace the old frames.
When inserting and deleting frames, if you click on the audio track's title box (at the left end of the track), you should see the red mark disappear - this means that the audio track will not have edit operations applied to it (note - this feature is not available when inserting a movie, only when inserting frames).