Hi
Audio and video can only be unlinked and relinked when from the same clip but not unlinked, repositioned and relinked in the new position.
Neither can you link unattached audio and video tracks, however.....
Once you have finalised the position of your video track and separate audio track for a section of the timeline, you can prevent them being moved by clicking the padlock button for the tracks concerned. This is the end button of the row of 4 just beneath the word "Storyboard" at the left hand end of each track.
This will essentially immobilise the tracks, so clicking the button for an audio track and a separate video track will link them together whilst still enabling you edit any unfixed tracks. The problem with this approach is that you can't now move them.
Unfortunately as far as I know you can't link two independant tracks and drag them about.
As an alternative, you could work with short lengths of timeline, set the audio against the visuals and save each as a sequence. This will produce new linked clips (sequences) which can then be used to recreate the timeline.
Unfortunately a bug in the program (v4.08) does not replace the audio track on the timeline when the sequence (any sequence) is reused although it will play when previewing. This bug has been reported to NCH
Nat